I'd let them know straight out that that's what you want for your son. We've asked my parents not to buy certain things for DS and although they might not share the same views as us, they're understanding of our style of parenting. We discussed things early on and I think they're actually thankful that they know we'll appreciate their efforts in choosing what to do. For instance, they gave DS some beautiful and artful books for Christmas and some handknitted and sewn toys, and then sent some money for his bank account at Easter (he's much too young to appreciate it, but they knew we didn't want chocolate).

Let them know early on- especially if they're there for the long run Then everyone will know what the story is.