Do NOT under any circumstances relinquish the teaching of your children on ANY SUBJECT to your parents no matter HOW good intentioned they are!! DONT DO IT!! It might seem like the easy way out for you but those impressionable young people will not want to disappoint grandpa and will do what HE tells them to do because YOU arent saying no to it! My mother in law allowed her youngest daughter to go to meetings with her JW elder husband even after she herself DAd to "keep the peace" with her husband. What can it hurt? She said... Well now that daughter is BAPTIZED, sneaking off to meetings with the HUSBAND and refuses to go to any other church with her mother or do any holidays because she doesnt want to disappoint her Step father!
You CANNOT unring that bell my friend. Just tell him that teaching your children on ANYTHING religion or not is YOUR responsibility and that you would rather not start that precedent.
Plleeeeeeeeease dont let him do this.