I agree...I think just a simple thank you note would be sufficient. Maybe you could just mention "I hope you enjoy the accommodations we have chosen" and leave it at that...don't mention that you've paid for it...like you said, they already know that! If you're anything like me you'll want some kind of favor with your names & date on it so maybe you could do the letter as a scroll buy personalized ribbon to tie it? It is relatively inexpensive. If not, I think just the letter is sufficient. Maybe arrange to have it in their rooms when they arrive? If you really wanted to get into it you could make all the letters personal. "Dear Aunt so & so & Uncle blah blah, thank you for coming all the way from whereever you are from to celebrate this joyous day with us."
Hope this helps some!