Considering the fact that your children can’t get a true perspective on God in a school that excludes Him, there is a better question: “When should they not be in Christian school?” If at all possible, have them taught from a Christian perspective every day of their lives. As Solomon writes, “Train a child in the way he should go, and when he is old he will not turn from it” (Proverbs 22:6).
Now, if you’ve placed your children in a public school, you are not guilty of a sin. And unless God has expressly forbidden it in your case, it isn’t sin at all. The question is not, “Is it lawful?” but “Is it profitable?” Your task as a loving parent will be much easier if your kids are placed in an educational environment that reinforces biblical principles, rather than refutes or ignores them.