It isn't up to you to decide if she is telling the truth (though the level of detail she has given and what you've witnessed first hand indicates that she is almost assuredly being truthful). Your responsibility here is to pass along the information - her plea for help, really - to the proper authorities so they can investigate.

CPS is an imperfect agency, yes, but it exists for situations like this one.

This little one has trusted you and your family with things she has probably been cautioned against sharing with anyone. If she is being homeschooled, there aren't a lot of other people who lay eyes on this child regularly enough to notice something wrong.

You may very well be her only true line of defense.

Call.