I agree with Stephanie, and was going to suggest that you lay things out to begin with. Ask them, point blank, but tactfully, what they want you to help with around the house. Will you be responsible for doing some of the cooking, laundry, general housework, or will the wife be doing it like she always has? How do they want you to discipline the kiddos while you're alone with them during the summer? Are there some things that they specifically DON'T want you to do? What part of the financial needs will you be expected to help with? Those are all things that I'd want to know, if I were in your shoes.
I think it's a great opportunity for you to get to spend time with the grandkids. I wish my parents were closer (they're in a different state), so my kids don't really know them that well. I think I'd look at it as a blessing, and just make sure you know the groundrules going in.
Let us know how things go!