If you feel you could go through the stress of the process, you could apply for a Child arrangement order. Cost about £250, starts with a thorough process from CAFCAS (children & family court advisory service), where an experienced social worker will be appointed to talk to yourself, the parents and most importantly.,the children. They will take their findings to the court - where you could represent yourself or hire a solicitor or similar. The results should always be “in the best interests of the child/ren.”
Easy to look into online, but quite a long and stressful process, so definitely not for everyone. There is also the possibility that it may annoy the parents even more which is a common reason for many grandparents not going through with it.
My heart goes out to you, you just feel so helpless don’t you? To not know the reasons why is even more hurtful. It just seems that if we don’t fit with their expectations....we can be “cancelled “, so, so cruel.