I might have invited the “stay at home” mom in for a conversation and told her that if the little girl stayed there she would receive regular religious training including instruction on the Theology of the Body, God intended men and women to be together not woman and woman. Discussions would be had about her biological father who is out there somewhere. If “mom” still wanted her there then she would have to sign something saying that she was okay with this course of study.
That’s basically how my kids’ Christian school handles it.
When the kids get older, they cannot take a member of the same sex to dances. They cannot publicly be in a same sex relationship.
They have to take Bible classes and go to chapel.
When anyone enrolls a child in a Christian school, they should assume the child will receive religious instruction. I don’t think the school needs to give them special treatment on this matter. They should just go ahead and give the girl a proper education, including religious instruction, and age appropriate facts about the birds and the bees. Eventually the lesbians will figure out the girl is getting an education they don’t approve of, and they’ll probably withdraw her, and act shocked, and maybe even sue. But such a lawsuit is bound to fail. And in the meantime, the little girl will have been given a foundation of truth.