K.G.
My parents tried that with me. My aunt and grandmother did, too. Especially because it is your house, here are some things that have worked for me with my boys:
1. Make light of it. "Grandpa, you're already big and strong, so you might not need to eat all your food. But M needs to so he can be like you when he grows up!"
2. Refer to what your daughter already knows. "M, what time do you normally go to bed? I know that it's special now that grandma and grandpa are here, but if you want to be ready to have fun with them tomorrow, you need to go to sleep right now."
3. Not fun to do, but over-ride their "suggestions." "We have worked very hard to raise our daughter well. You seem to like how she's turning out so far. In order for her to stay on this track, she needs to _______."
4. Pretend to be hard of hearing. "I'm sorry, I thought I heard you say this. I'm sure that's not what I heard, because it's not best for M."
5. Not easy, but consistently ignore or pointedly talk against all his ideas. "I know that grandpa said you could watch a movie, but it's a much better idea if he would read a book to you and then tuck you in bed."
Here's the deal: you are the mommy. Your husband is the daddy. You get to say how to raise your daughter. My parents (my dad especially) tried that with me, but it was a no-go. I shot him down pretty quick, and now he respects my rights as mommy. It was really tough for me to speak up, but I'm so glad I did!