Seems a very serious problem. U cannot discuss with them as it won't go well. Hence the only one who can talk is ur husband. He has to literally sit them down and talk about it all. Esp he needs to tell them tht their behavior is disturbing the kids and they may completely lose the love that they hav for grandparents. This discussion needs to define some ultimatum. And whatever is decided upon you and your husband should abide by it till a better solution is presented.
For kids to slowly get over this feeling of alienation try to make them express their feelings over drawing them. Ask them to draw how they feel about going to grandparents home. It will help u guage their true feelings.
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