Unsure how to handle older children with my toddler

I was first thinking this happens since 19 month old should be supervised closely and protected against older toddlers/preschoolers, but the age of the "older child" is concerning.  Frankly a 6 year old should know better.  I am not surprised the parents did not do anything, they likely did not notice, as very few parents closely supervise their 6 year old on a playground.  I watch my school aged kids to make sure they remain on playground but don't pay close attention unless someone is crying/falls/or gets my attention, since my attention is focused on my 2 year old.  My kids are very gentle with younger children and even though they will get into fights and confrontations with other kids their age (it happens to most kids), I would be shocked to see them be rough with a baby or such a young toddler.  If it was an accident, I would ask the older kid to be careful, if on purpose I would tell him that your son is a baby and he/she should be more careful around him, not hurt him, and wait his turn.  If that does not work, I would loudly say ("excuse me, who is this boy/girl's parent?") and then go talk to the parent.  I have done that before when a kid was very aggressive and as mean as it sounds, it is not my job to discipline other parents' kids -- I'll first try to explain to the kid once and re-direct but when that fails or keep happening I go and get the parents involved.  

I wouldn't hesitate to speak to a child that was acting poorly toward my toddler. And by speak to a child, I mean speak for your toddler: "His name is Adam. He's not really a baby anymore and he wants to play, too. Would you please share the path?" or "Excuse me. Please don't push him. He's having his turn."

I have both a 4 and 6yo and admit, I'm much less on top of them at the playground/park these days because they need that freedom to play, too. But certainly not at the expense of your child so I wouldn't take offense if you spoke to them in this way. And if they weren't respecting you or your son's wishes, I'd certainly thank you for bringing it to my attention. 

I truly feel your concern. Although my kids are much much older now, I faced such situations in the past, especially since they were relatively shy girls. I decided to stay very close to the girls and speak directly to the older kid (and for all to hear) things like  'no, it was her turn to play now' or 'please wait for your turn'  or 'could you please move out of her way'. It usually worked. 

I am dismayed that parents of older kids in a mixed age setting sort of let go of supervision and assume they are 'fine'. It happens all the time and I am sensitive to this too. Some of this behavior when repeated and unchecked, perpetuates to 'bullying' later in life, and we would not want that.  

When I had my first kid I was pretty shy about telling other kids what to do, but I got over it quickly! There are a lot of kids out there who just don't know how to act and don't have anyone telling them "no." I'm all for letting kids sort things out for themselves when they're all the same age/size, but at 19 months you are definitely within reason to model strong behavior for him by gently but firmly saying to the other kid things like, "He was using that," "You can have a turn after him," "Please wait for your turn," "Please be gentle with smaller kids," and other forms of "what you're doing is not okay." Give him a chance to try to solve it first, but it will help him learn to advocate for himself to see how you handle things. I was always very polite, definitely didn't touch the other kid, but I didn't take no for an answer, and I wasn't above asking kids where their grownup was - usually just the mention of that was enough to make most kids shape up, although you'll still meet some kids who just truly don't care. In that case, you can play with your toddler to provide a buffer from the aggressive kid.

Ugh. I’m sorry outward going through this. In this day and age of helicopter parents are those parents not hovering nearby? I would totally intervene (“Excuse me, I’m sorry, no he had that toy first/can you please let him by he’s a baby.... You can wait your turn/share etc.” loudly enough as a- a way to model self-advocacy to your child and b- embarrass those parents for not managing their kid better. I suppose you should give them the benefit of the doubt and assume they didn’t observe the behavior. 

I’d be mortified if my kids acted like that. 

Those "older kids" are still little kids and you don't need to say anything to them or their parents. Simply protect and empathize with your child. The six year old name calling and blocking, simply go over there and help your child navigate away from the other child either by picking him up or taking his hand and leading him away and empathizing: "You seem upset by that other child. Let's play somewhere different ". For the 4 year old who pushed in, empathize with your child: "you were there and the other kid wanted a turn. He pushed you and you didn't like that."   

In these situations I think it's fine to admonish the other child. For example, "Please stop teasing and blocking my son" and "Excuse me, my son wasn't finished, can you please wait your turn?" It can be uncomfortable, but it's important for your child to see you standing up for him while he's still too little to do it for himself. If the behavior continues after you say something, I think it's fine to look around and ask in a raised voice, "Who is here with this child?" and then speak to the adult, calmly explaining, for example, "My son would like to play here but your child is blocking him, can you help us please?"

Not sure if this is right thing to do, but what I do with my two year old, since he hasn't "learned" to stand up for himself yet or what to do in these situations, I stand up for him so he can see how to appropriately deal with difficult situations. For example, my son was "pushed away" while playing with a steering wheel at the park so I stepped in and said, excuse me, my son is in the middle of playing with this, when he is done it will be your turn and navigate my son back on the toy and the other kid off. Of course if my son is monopolizing the toy, then after a period of time you would say you have had your turn and this child is patiently waiting for his turn so now it's time to do something else etc. My opinion is that you want to show him the behavior/what he should say/how he should react that you would want him to take so then he can learn and model it later. I've found its easy to step in right away since the older kid knows he did the wrong thing and that he can't get away with it just because your kid is younger. That's what I do at least but I'm curious to hear what other people do too.

It’s so hard! I like Janet Lansbury’s respectful parenting blog for stuff like this, she has a couple of podcasts on helping your child navigate social situations without too much hovering or letting your own indignation influence the interaction, etc.  

At the playground, you just go up to the six year old and say something like, "That doesn't seem like a nice way to treat a little kid. Can you be nice instead?" If his mom comes over and gives you crap, you go, "Oh! There you are. I thought he was here alone, since nobody seemed to notice he was teasing my kid." 

At the museum, you just move on, because people are impossible sometimes. 

I think a good rule of thumb is to speak to the older child the way you would want your own child spoken to - because he will be a 6 year old playing in a playground with younger kids one day (sooner than you think) and may also need to be reminded about how to behave towards littler kids!