Academic Enrichment for Public School Kids

We are in El Cerrito so a different school district, but this probably still applies. A couple thoughts. You don't need to test for giftedness. The district determines that and will send a letter at some point in 4th grade following the 3rd grade assessments. WCCUSD was very behind on this this year as they were changing their criteria to give more weight to teacher evaluations relative to test scores, but scores are still a big factor. Anyway, we just got the letter for my 9 year old rising 5th grader. But, that itself means nothing as our district doesn't fund GATE and we knew our son was advanced in certain topics anyway. I had previously (when he was midyear in 3rd grade) gone through the CTY testing process with him with the same thought you are having, an online class from home. I still think this will be great - but for schedule reasons, we haven't committed to it yet. I'd strongly advise you to consider it - we are going to try to find the time for it next school year (balancing it with "fun" afterschool pursuits). I would also strongly suggest making the time over the summer for your older one to do the sleepaway CTY (or TIP through Duke) academic camp. I did this when I was a young teen and will absolutely have my kids do them too. We're trying ATDP for the first time this year and will see if it's as good as I recall CTY being for me.

Aside from spending the big bucks with CTY, our kids (also have a rising 3rd grader who seems on the same trajectory as her brother) have done Khan Academy missions for the next grade up, and we have a lot of tinkering/art supplies on hand for free play and exploration. Reading of course but as you say that is not a challenge per se for some of these kids. I anticipate the older one will probably tap out in grade-level math curriculum in junior high and start trotting over to the high school for math, and in high school will probably take math at the community college. We'll see. The younger one already skipped a grade so that has helped her not bump up against boredom yet (she just finished 2nd grade). I also have them enter lots of "contests" (county fair, Scholastic contests, etc.), for project-based challenges but also to get used to the idea that they AREN'T always the smartest kid in the room, and they will lose sometimes. We are more concerned about them developing grit and resiliency than always being academically challenged in everything.

Finally, in the classroom, both their teachers this year recognized their advanced skills and made a real effort to differentiate some learning, which I appreciated. I also recognize they are not advanced in everything, and there's plenty for them to work on inside the classroom. So of course I advise connecting with the classroom teachers at the beginning of each year.

I am in a similar situation though my kids are a bit younger (early elementary).  I did look into private schools but the ones close to my house are not that much better academically -- they differentiate themselves by smaller class sizes and more attention to emotional well being of child and more art offerings, but as my kids are very social and love being part of a large group, I did not think there is value in private school for my child.  We are sticking to public and using the money saved by not paying private tuition to pay for enrichment at home.  Here is what we do so far:  (1) we bought memberships to museums (both art and science based) and go regularly and spend the time to really explore with the kids, i.e. we read all of the signs/labels/notes, do the experiments, discuss the how's and the why's, and carry a notebook to take notes and make predictions prior to experiments and write conclusions, etc.; (2) we buy books that are at their level and focus on non-fiction that educates on topics they are interested in (though it seems you already do it); (3) we buy those educational experiment sets to do at home and ask them to keep a notebook of their work (they love it), we also buy those electricity, robotics and other science and engineering sets for home and let them build and tinker with them, which is great for their science and math learning; (4) we are paying for several apps/computer programs that are great educational tools for home, i.e. IXL is great, my kids also love Khan academy videos, and other apps and computer programs (my kids each have a device (computer for older one, a tablet for younger one) and they are allowed unlimited screen-time for educational purposes.  We considered a tutor but did not find anyone who was good at simple enrichment, as most tutors focus on helping those lagging behind or helping get advanced in a certain subject or prepare for a test, and I was looking more for helping my kids learn by doing and learn that learning is fun.  My kids are very self motivated so providing them the tools and resources and letting them be works best for us.  Both parents are highly educated and we do spend a lot of time with the kids in evenings and weekends discussing their experiments, helping explain certain concepts that they did not fully appreciate from online program or by doing the work themselves, discussing books they read, etc.  If you can find a good tutor to take that role that would be great, but I could not find someone so we are combining family time with fun learning time. 

If your kids are interested in math, consider Berkeley Math Circle, http://mathcircle.berkeley.edu/  The coursework is very challenging and far beyond what is taught at school.