I probably wouldn't charge either way and I would probably do an outdoor playdate in the back yard or something like that or even meet at the park if you are worried about having someone in your home. Then suggest that she drop her son for a few hours on his own at your house before leaving him for the full day. Although as a parent, I have always done what this woman wants to do before having my kids start full-time, so I do allow parents to come by. To me it would not feel right to not allow them to see the kids playing together etc. Maybe I am too easy-going though...