We are going with the Gran Bahia Principe Tulum as we have had some friends married there and they are great things for kids. We'll have kids there from age 18 months to 10)
In terms of your guests going for only 3-4 days and you and your FI staying the 7...check out the price difference with your travel agent...when we starting at looking at places to have ours it turned out the4 day package was just as much as the 7 day and when you factor that you are travelling on your first and last day it turned out not to be worth it to book 4 day packages for our guests....at least for us anyway. Not sure if any other ladies have run into the same thing!
Congrats though and happy planning!