How well do you have to know another parent before you leave your dc there alone?

[Travellerintime]

DD (4.11) has been invited for playdate at new schoolmate's house tomorrow.

Have only met her mum twice, but I like her, plus she's a friend of a good friend, so I feel she's a known quantity. Therefore, I am happy to leave her there for playdate.

But it occurs to me that other playdates may happen where parents might not really be known to me or other friends - what do you do? (should point out that dd has just started school and we don't know many other parents/children there yet, so this is a possibility)

I tend to trust that they are, on balance, unlikely to be child abusers, and just let them get on with it.