I'd definately follow your gut and ask. Like you said, this is someone who is essentially with your son (at least in the same house) so you really do have a right to know IMO.

Well, thats how I'd feel if it were me.

It might feel a bit intrusive asking "so who was that here when I picked up DS" but, well, your number one priority is your DS, and if looking after him means asking questions about the person who is looking after him 2 days a week then thats what it means.