Number one of course would be to try to stay in the same resort, and request rooms close to each other.
The Pirates Cruise that disnut8 mentioned is a great idea, very popular among the littles.
Matching t-shirts wouldn't cost too much; lots of people design iron-on graphics for their vacations and put them on inexpensive shirts from
WalMart for the whole clan.
Plan on some group touring where both families spend a few hours together enjoying attractions.
Plan on some time when your DS goes touring with them, and their DD goes touring with you. This will give the kids time to have fun together without forcing the entire group to stay together.
Plan one or two meals (maybe one when you arrive and one just before you leave) where both families dine together. Crystal Palace or Chef Mickeys are good places (both character meals).
Give both kids a chance to pick their favorite attractions from park maps or from the vacation plannind DVDs, then match up the lists to see which attractions they will most enjoy together. This will help you to schedule the families' together times and seperate times.
Try to keep things casual. The kids will enjoy each others' company, but the more you force things or fawn over how cute they are together, the more annoyed they will get, or embarassed, and the more likely they are to get grumpy about the whole thing. Keep your attitude light and don't try to force "togetherness" activities.
Above all else, remember that it's a vacation - it's about having fun.
