Child support from American citizen

I need advice.

The father of my children works in california since 2007. I broke up w/ him of some situations that i can’t handle anymore like “he wants to get married there” and now he got married there last december 2009. and now he’s american citizen. he never gave it the tax refund annually for our children. he asked me to changed the surname of our eldest son to his surname so that he can less the tax and to save for the future for our children as well, so i did it. though he’s supporting financially monthly. Do i have the right to demand for that tax refund for our children? how much financial support for the children should i get to him?

your reply will be very helpful for us. thank you and more power.