They are actually bullying you and totally undermining your role as the childs mother, it’s pure manipulation and so disrespectful to you. We’ve all given in at times for a quiet life but ultimately we then make a rod for our own backs. If I was you, I’d put the kids to bed and then both you and your husband together, a united front, should be calling them in for a meeting, and laying out some stringent ground rules, before it’s too late. If they give the child everything she wants, of course she will want to stay with the grandparents. You must learn to be assertive, as you in turn will need to teach this to your daughter so that she stands up for herself in life. You can’t help it if you are kind and soft, however, you are making your own life very difficult .... I wish you well.