Ooh, I agree with Llyn re the screaming kid. Scream back. I have done this in the supermarket, and the look on the kids face was priceless. When the dad started to sound off, say "it's okay to scream in here. Your child was doing it, I just thought I'd join in". He can b***h all he wants, he's an [censored] anyway and you'll never change his mind. But if the kid gets screamed back at a few times he'll stop doing it.
As for the wanderer - call Child Services. Say he just appeared and you have no idea where he comes from. Once mommy dearest has had to explain herself to the authorities, she won't be letting him wander too far...
Argh. Some people are so so moronic it makes me want to barf.