If they haven't flown before, give them a little coaching on what to expect at the airport - TSA screening and all of that. DS has been traveling since he was an infant and when we brought his buddy along the first of several times, it was maybe his second flight ever but first flight since he was pretty little. I hadn't occurred to me to let him know what to expect, and it would have made it a little smoother at the checkpoint if one of us would have clued him in to putting stuff in the bins, etc. Even what and how to pack a carryon, stuff we take for granted if we travel a lot.
Gift cards have always been a great way for us to let them be independent with buying snacks and counter service meals. If you do GCs, register them ahead of time so you can quickly transfer funds around if they get low or someone loses one, etc.
Depending on age, separating and giving independence can be really fun. We started the first year with them maybe riding a different ride than us, or getting food somewhere while we did something else, little bits at a time. As the years progressed we eventually started hopping to different parks. Of course, DS knows WDW like the back of his hand, but if they're outgoing enough they can ask directions very easily.