I would suggest this is actually a safeguarding issue and that Early Help might be the best way to enable the parents to access better support. It’s not a judgement on their parenting – more a necessary step to get the world to acknowledge the difficulties they face as a family.
The PDA society may have resources which are useful.
The only way to lower the anxiety is to lower the demands, but it’s very hard when you’re the parent and supposed to be in charge.
There are some good books on Amazon written from the parent’s perspective and I would also put mum in touch with any online or face to face support groups specifically for PDA. The NAS may also be useful here.
As you probably know, educational outcomes in PDA cases are not great – we are still, in Barry Carpenter’s words (though he was writing about FASD) “pedagogically bereft” for many of these children..