Little update. Spoke to my son last night and he would firstly like to thank you all for your very valuable advice and info yesterday. He rang our GP last night and has arranged to go there to fill out forms to have the children registered there, as well as remaining at their existing GP. It makes sense anyway, incase they need medical attention whilst staying with us. Then he is going to book a day off to take the eldest for a checkup. We can get the sores checked out and discuss her speech problems.
We have decided, whilst a bit confusing for the girls, we will keep fresh clothes here for when they stay. And only send the laundered ones back and a few new ones occasionally, as although my son pays maintenance, as a Nana, I would want to treat them regardless.
Then we will monitor how they are thriving and if we are still concerned, we will arrange for a HV to go to the house and speak to the mother. I think rather than jump in with both feet, causing a lot of bad feeling, upheaval and maybe a court case (which we would do should things get worse of course), this more softly gently approach is the way forward.
I think the most valuable thing we learned yesterday, is he has rights. So now, armed with that information, at least if things are not being done properly, he won't be so frightened to confront her and insist she acts. And if she threatens to stop contact, he will be able to stand up to her.
Thankyou so much all.