For both sides of our family, it was always an issue. They couldn't understand why I was so rigid in our routine and so clear with all of the family members on how they should interact with our kids. I had a much harder time with them than I anticipated. They defied, mocked, changed routines and sometimes I think they did it just to see my over-reaction.
With the in-laws I just said, "You had the chance to ruin your kids, you have to give us the chance to ruin our own. You are the grandparent, I am the parent..respect that." Yeah, I get a little harsh when I feel disrespected and bullied. If one chooses to say it with a light tone, it doesn't come across so bitchy. I heartily suggest light and quippy when tossing out those statements.
With my parents it wasn't so overtly confrontational since they used the "you weren't raised this way, why are you rejecting us" tactic. Not nearly as pleasant as what I make it out to be actually. I took a bit more time to argue my point, but in the end, they didn't agree, just shook their head and predicted a horrible outcome.
I have to say, standing my ground with them was the best way I could have handled it. Now, when they are really enjoying the kids and are remarking on their progress, behaviour and personality, I get to have my 'I told you so' moments. Believe me, I take 'em. At the time, my DH cringed when I was so confident and vocal. Now though, he chimes in when anyone even thinks about altering our schedule or suggesting that we relax a bit. I think having the only consistantly well behaved and respectful kids on both sides of the family really helps though
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Anyway, we were direct and set the parameters for all the people coming into the house. Sure it was difficult, but they all respected our right to do it even if they didn't agree with the reasoning.
K.