My ex's parents tried something very similar to this after I left the position of babysitter vacant. It failed spectacularly for all involved.
I've found it best to keep a distance between me and them. Our only contact is about their grandson. I never even mention their son's name, not even to say that I heard from him or whatever. Better that way, as I am working Alanon recovery and they are still deeply enmeshed in his issues.