OP, do you think there is a way to open up a discussion with these moms?
Well, the first mom is handling things well IMO and has been trying to rectify this behavior in her daughter for some time. She says her daughter became this way in part because the older kids at her daycare were very tough and she had to learn to defend herself. But instead of becoming strong and assertive she turned into something of a bully herself. The mother switched her daughter to another school where they have been working on this problem, but she still reverts to bullying when she's tired, etc. Her daughter particularly likes to walk past another toddler and push them until their head hits the floor.
The second mom is a SAHM and this is her first kid and I wonder if she just doesn't know how to react as a mother, but I also think she thinks her son is an 'alpha' male. Watching your kid do this stuff over and over and not reacting just baffles me. I think in her mind she might be 'blaming the victim' (he's smaller so my big son can't help himself). I do have to say that although my child is no angel, these 'attacks' are unprovoked and are not due to not sharing, whining, etc.
Since I have older kids I've witnessed this kind of thing before...A child might misbehave in a social setting and the mother gets kind of embarrased. She usually doesn't like it. Instead of putting their foot down and just correcting the child (as ellemenope suggested) they get defensive and somehow try to blame the other child even when it is obviously not that other child's fault. I think the mother's love for their child causes them not to see their child's behavior for what it is. They would never like that behavior in somebody else's kid.