You are living in Japan in a bicultural, bilingual relationship (meaning that you can deal with the dry-cleaning guy in Japanese). Little Tomu or Tommy, your first, has gone from goos and gurgles to words and even sentences. How cute! Kawaii! You, who have struggled so hard to master Nihongo (or at least understand the dry-cleaning guy when he comes to collect), are amazed, pleased and more than a bit envious that this pint-sized wonder is picking up the language so quickly. You want to help him along (or fear that he is going to overtake you), so when he prattles in Japanese to you, you prattle back.
Don't.
That is the lesson I learned more than 15 years ago, when my son started talking. I wanted to raise him to be, as I was not, truly bilingual, but I thought I could do it pretty much the way I communicated in my marriage -- a bit of English here, a bit of Nihongo there. But I soon learned to my dismay that, instead of my own brand of chanpon, the kid was replying to me in pure, if not quite fluent, Japanese -- and nary a word of English could be heard. The reason was simple: I was outnumbered. His mother, grandmother, nursery school teachers and friends were speaking to him in Japanese and so, in a way, was I. What did he need English for?