We have the same problem with our grandson, his behaviour is awful. Cheeky, out of control, insolent, running about and jumping on furniture, pouting, going into meltdown (he is 9 by the way)
It was put gently to us recently not problem with GDS but DD who I am afraid to say is only interested in herself and own life. It's so sad. We have tried speaking to DD but goes on the defence immediately and won't discuss the problem. We have got no further and I really do feel for you Muddigran, we are all stuck in the middle here. I dread the teenage years with GDS, the problem will only get worse. DD did go to parenting classes for a few months and there was some improvement but now gone. Muddigran you are welcome to sent me a private message. Keep your chin up and remember they are your grandchildren.