Hi Karen,
I think you are going to have a wonderful time at the Walt Disney World Resorts this April. I do not think the age is such a big deal. I think you need to look at your children's interest to determine if and when you will need to split up.
When my daughters were seven, they would go on any ride that went fast - Big Thunder Mountain Railroad, Splash Mountain, Rock 'n' Roller Coaster. However, we have friends whose son is 11 will not go on those rides. Since your son is 13 and has never been to Magic Kingdom, I do not think he will mind a trip on Peter Pan or Dumbo at least once as I have a husband who does it once for the family.
I would definitely work as a family to plan this trip. You can order a planning video or go to the Walt Disney Resorts Theme Park homepage at
http://disneyworld.disney.go.com/wdw/parks/parkOverview?id=ThemeParkOverviewPage. Have your family make a list of must see at each park and then agree as a family (this is something my family does) what rides everyone could do together.
My eldest daughter and I are not fond of Tower of Terror. But we will ride it once as a family. When my youngest daughter and husband go for the second or third trip, my other daughter and I get a snack or go on Rock 'n' Roller Coaster. Splitting up sometimes allows some special one on one time you normally may not get during the busy school year.
I think with a little planning you are going to have a great trip! And remember, it was Walt's main reason to build this magical place so that families could spend time together.
Beth