This is just awful OP. I am a grandparent carer who has a residence order for my GC. This is nothing for you to worry about in itself, my circumstances were very, very different and I certainly did not actively seek to take them.
She has been quite clever in establishing herself as a prime mover in your GS's life. Overnight stays, taking him and the other children to appointments, collections from school etc. But she does not have parental responsibility and never will have. She has no right to take them to any appointments of this nature and had no right to take your son. You need PR to give permission for medical treatment etc.
You need to speak to the school about collections to ensure they are not handed over to her without your permission.
She could, worst case scenario, gain contact with the children by applying to the court. However, she cannot just apply, she would have to apply for permission to take proceedings in the first place. She would have to go through mediation first as well, all very time consuming and costly. Do remember that many people cannot afford representation in family matters now and do self represent. At least you get to say what you want to say.
It is not my intention to worry you but you really need to take some legal advice from a family solicitor (30 minutes free interview), speak to the school, doctors etc.
I believe she is very foolish to have upped the game in the way she has although I doubt you will see it this way at the moment.