If it were me and my HUSBAND couldn't lay the law down on HIS mother (get where I'm going with this?) I'd put a big old smile on my face when she came to the door and say "Welcome!!! So glad to see you!!! I was just on my way out. I'm so glad you and your son will get to spend some alone time together. He misses you so!" and then walk out the door with my bags packed to spend some BIG fun with my best friend at her house. Your husband, left to the kids and his mother will get the message and so will she! That's what I would do but I'm not you.
Call me evil but YOU run that house, not her and obviously not your husband. TAWANDA!