[gacba]

As a parent of 3 girls, I can't emphasize that trust thing enough above.

I would absolutely, never, EVER use such a service. My wife would balk at the very suggestion of it. It sounds like a great idea on paper but no matter how much profiling, checking, due diligence, etc you put into it on your side, none of that matters to her, or my kids. They have to KNOW and TRUST the person they are with. We need to be able to vet that directly by observing them with the kids and get references from people we already know and trust ourselves. Having someone else say "Yep, they're cool!" when we don't know the source, means nothing.

Most of my friends who are parents are equally paranoid about these issues in this day and age (sexual predators anyone?). You're going to have a really tough time selling this to your target market.

You may not agree with my perspective, but I can absolutely guarantee this will be your #1 roadblock in getting parents to sign up for such a service, so you need an iron-clad answer to that question at the very least.