potty training

That's such an outdated method of potty training now. Positive reinforcement is better. Reward the children for a successful outcome and ignore the rest. Children with developmental delay will need more patience. You can't interfere but could lead by example, stay calm and help if asked. Good luck.

I can’t think of anything worse- degrading, humiliating, unkind and guess what? More likely to give them hang ups about going to the loo/potty training than anything else I can think of.
You have a real problem - her cultural background will be hard to overcome, but in my goodness, those poor little boys.

What I did was buy a couple of extra potties from the op shop and have one in each room.
They also had a child's seat which fitted over the toilet for poos, although they could use the potty if they wanted.

I don't think you'll change your daughter; she has a hard job rearing two little ones with developmental difficulties.

I wonder if she has regular professional advisers who might get through to her that she is doing far more harm than good?

Otherwise, all you can do is point out to your son (who must have seen this angry behaviour) that these wee boys have enough to cope with, without an angry mother making them worse.

If you stay with them, just tell them what good boys they are; praise them and encourage them in every achievment. Be a haven for them and reassure there is nothing bad or dirty about toileting.

Every living thing gets rid of waste one way or another. Maybe you can turn that into a game?

Hippo poo and giraffe wee-wee; footballers, singers, pilots and puffins - all do it