Thanks for the suggestions. I will NOT be providing a babysitter for the night. It's only this one cousin that has kids (my other 1st cousins are all in their 20s and not married). I was thinking of wording the invitation "Together with their children and grandchildren, Mr. & Mrs. Blah Blah invite your to celebrate their 50th wedding anniversary". Then at the bottom put Adults only please. Like I said it's really just the one cousin who this would apply to.
No no no no no. that will just totally confuse the issue, and the cousins will surely think that their kids can come since the grandchildren will be there.
Just keep it simple.