It was interesting reading all the responses. I had to actually reread your post as I thought about this.
When my 4 and 2 year old boys go to my parents to stay, I DO expect them to be parents to my boys. I am leaving them there under their supervision for a period of time. Being raised by my parents, I do know their techniques of parenting pretty well. I also make sure that they know the differences with my boys. For meals, they do have them try new foods, but we do that at home too. I do expect my boys to follow their rules while I am not there (just like in school, etc..) or I should not leave them there without me being there.
I would talk to your mom. You know your relationship with her and how to approach her. Your kids may be telling you the truth or an exagerated version of the truth or pieces of the truth. I already get "different" stories from my boys and they are younger than your kids.
Open communication is the best way to go. If you and your mom can't get on the same page, then your kids shouldn't go there alone.
Your parents have raised kids...they actually have more experience than most of us. They are also human and operated in a different time period. It all plays together for experience.