RE: A Battle of Languages-- Raising Bilingual Children
It's about exposure levels. If you are concerned about an imbalance, try to for example bring videos and games into their environment that balance more towards whichever language you want to help them develop more in.
Also, maybe consider changing how the education process is going to go. Homeschooling you can control the language exposure and you can eliminate the nonsense of teasing from language difficulties. Your son focusing on Japanese is clearly the result of shame from his lesser english, and the predominance of Japanese in his time spent in kindergarten.
I assert and I will say this until the cows come home. Socialisation should not be forced on people. Period. Education in groups should be as consensual and voluntary as any social activity should be. Otherwise there will be torment. It's intentional in the design of State regulated education, to teach conformity. Nothing to do with learning.
Exposure definitely is the key, and right now the only opportunity for my son to speak English, for the most part, is with me. Of course, I have my children skype with my family weekly and we get together with other international families when we can, but when it comes to speaking opportunities, my children face an imbalance. You're right that homeschooling would help. If I can find a way to work from home, that would be an option for me. Unfortunately, right now it isn't. I disagree with the shame part of your comment. I concede that it very well could be a possibility and a factor, but I think my son is A) working with the language tools that he has, and B) fighting to have more time with me. I think he views his sister, and her language acquisition as a bit of a threat. This is something that I am trying to find a solution to as well. My son also knows that I speak and understand Japanese, so he has me marked as a mixed speaker and can sometimes be lazy about which language he uses in front of me. Thanks for your thoughts.