If you're really bothered and sure there's a problem you should knock on and let the mother know you can hear all of this. If she really recognizes the malevolence of her ways then she'll stop and if she doesn't well at least you can report it in sound mind. What's likely though is that she'll get highly offended and slam the door in your face and tell you to fuck off, but at least you have the conscience of not going behind her back when you make the next steps.I've got a neighbour who's constantly threatening to 'twat' her kids and beat them up for the most minor thing on a daily basis pretty much. The thing is, I've never seen the kids with injuries and they seem pretty normal, albeit sadly likely to turn out the way their mum is. Nearly every time I'm in the garden I hear it. What should I do? The kids are maybe about 12 and 10 years old.
Although, if at all possible, maybe it's worth getting to know her and the situation at hand if anything. Maybe she's living an extremely stressful life and needs support more than anything. Maybe she'd never harm her children but she just uses language her parents used with her and she'd think it's normal; maybe it's her equivalent of threatening to take the toys away. I'm not saying threatening to hit your kids is right, but maybe there's more to this. Of course, she could just be a nutter and these kids really are in a bad position.
If you're absolutely convinced there's a problem then address it directly; either with her or if you think the kids are in real imminent danger with the authorities. But you need to be sure because otherwise you'll be putting an otherwise innocent family through a shitstorm in an already rough time. Otherwise do some reconnaissance and learn a little about it all. Invite her in for a cup of tea.