I am married with a woman from Holland and after that I discovered that she believe in Buddha and I have a girl from this woman. I don't know what I have to do because I am Muslim. In the beginning she told me that she is Christian. Really I don't know what I have to do.
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
You should have inquired and investigated about the religion of this lady before you married her. If you find out that she is Christian, you may marry her although a Muslim lady would be a better wife, as she will bring up your children in accordance with Islam. A Christian lady might have a bad influence on them and even lead them to her religion.
Their end then will be Hellfire. May Allah protect us from it.
As for your marriage to this Buddhist lady, it is null and void and you must separate from her right now unless she becomes Muslim. So, any sexual intercourse you have with her after this moment is pure Zina (fornication or adultery). So, hasten to separate from this woman and do not leave your daughter with her as she might spoil her religion.
Allah knows best.
You can search for fatwa through many choices