I wouldn’t move even if money wasn’t an issue personally; you have a really good arrangement there unless there’s something important you didn’t mention. You get to save money on childcare and rent and don’t have to do housework, your son gets to spend time with his grandparents (especially important given his mom is out of the picture) and your parents get to spend time with their grandson. Why would you want to change anything here?
Sorry but I think your grandma is winning this one.