"if you don't want a child exposed to Christian teachings then don't put her or him in a private school that is openly based on such teachings"

It's sad that even that small a stretch of logic seems to elude such a BIG section of the population...

"how to resolve it to the satisfaction of both parties involved"
If the parent doesn't like what you folks do, why is the child there? I'd tell the parent to enjoy the fullest functioning of the egress...