I think that the Pinglet's English is pretty on par with kids her own age in the U.S. Sometimes I can see she has problems speaking in Japanese--like when she talks to Ping using katakana-style English (horrors!), instead of actually "interpreting" what she wants to say into Japanese. I know she doesn't have problems at school with this from hearing her talk with her friends, just sometimes when she switches back and forth. I would LOVE to be in her head for a bit.
Mini-Ping has had a bit of a language explosion over the past few weeks. He has a book that he loves, First 100 Words, that I picked up during a Costco run, which he takes out the minute the Pinglet gets back from school. He'll sit there and point at pictures, demanding that she tell him what it is. At the moment, his favorite words are "elephane" (elephant), "ilovu" (I love you, said as one word) and repeating everyone's names.
Both the Pinglet and Mini-Ping are very into music at the moment. Mini-Ping goes to a music Mommy-and-Me class 3x a month, and grooves around the room to the amusement of the other people in the room with their much better behaved children. The Pinglet has been begging to take ballet classes(?!?) and we've been considering it for next year. To balance out her love of all things classical though, she likes to sing along to "Tic-Toc" by Ke$ha, and occasionally asks me to define certain words, like "tipsy." (eek) I guess I could look at it as her wanting to expand her English vocabulary...right?