That's a big assumption.
Where do Missionaries go to preach the Gospel? To the First Baptist Church or to some a country full of atheists and non-Christians?
If the parents are that anti-Christian as to undermine all our teachings, they would not consider enrolling the child in our Christian school in the first place.
This mother may be a "do as I say and not as I do" type of parent. That is not ideal as a child should be taught by example. However, if the parent is willing to give her child a Christian education, the child should be given that opportunity.
I cannot imagine Jesus turning a child away for the sins of the parent.