Hi Paul,
Welcome back to WDW where the magical is bigger & better than ever! You must be super excited for your first trip with your kids. Seeing the magic through their eyes is one of the best feelings ever! Be sure to stop by Guest Relations to pick up "1st Visit" buttons for each of them.
As funny as this may sound, you can take your 6 month old on any attractions where she meets the height requirements. There are lots of them that don't have height requirements. For a list of attractions with height requirements & the required heights for all four of the theme parks, check out the
Traveling With Little Ones section on the WDW website.
Keep in mind that just because a child (this includes your DS4) meets the height requirements, doesn't mean they should ride it. For example, if either of your kids are afraid of the dark, avoid Haunted Mansion, Pirates of the Caribbean and Snow White's Scary Adventure in MK - neither of these rides have height requirements but all of them are very dark. If you ride the WDW Railroad, ride towards the back as the train whistle can often frighten young children. You know your children better than anyone though.
At 6 months old, both of my kids loved Its A Small World. The dolls and bright lights really drew their attention.
There are Baby Care Centers located in all of the theme parks. They provide excellent places for changing diapers, washing bottles, nursing, warming baby food and formula. They also sell a few baby items as well. Please feel free to come back with any further questions. We love spreading a little pixie dust on trips.
Wishing you many magical memories :-)