Comet is going through a phase where he gets aggro with some pups of around 5 months. Generally he is a very friendly little tike and he doesn't mind mixing it with much bigger dogs. Just something about pups that age I think.
Had a husky pup (huge) just charge at us the other day just being playful, but 25kg of dog running at you full pelt could do some damage. It jumped up at me and nearly knocked me over. Comet just ran around it and gave it a nip and it backed off. Owner was apologetic but if it had done that to a kid she could have had a problem. I think you just have to judge it.
A few weeks ago there was a dad playing football in our park (which is an ex golf course and massive with dog walkers) with his two kids (both 10ish). Comet ran up to them, seeing the footballs and the kids all tensed up. He didn't jump up or bark or even touch them and quickly lost interest but clearly he just wanted to play. I apologised, as you do, and the Dad started giving me aggro which he got back with interest (wasn't in the best mood that day.) After that I got a bit nervous with him off the lead. Later I changed my mind - the guy was being a dick for no reason. Some people just are.