how old do kids have to be to be questioned

What state are you in?

My first inclination, by the way, is that the questioning is probably allowed (and telling your daughters is likely to be immaterial), but I am unsure. Have you spoken to an attorney/Legal Services lawyer about this?

we live in wv

At any age when children are able to communicate and understand, CPS can question them without your consent, knowledge or presence. The idea is that a child won't be truthful when the abuser is present, so CPS usually questions children at school, daycare and any other place where the parent will not be present.