Thanks for all the replies! I'm actually okay if a stranger sits with my older one as I'd be right there. I just wanted to prepare him for it. So glad this will work out!
Great. It really is no big deal. Personally, I'm not sure it is even worth discussing with most kids, unless they have a specific concern.
HOWEVER, one thing you may want to take a few moments to prep OTHER logistics of the trip. One parent with two kids does poses a few (potential) challenges.
1. Luggage. Personally, I'd prep everyone to lend a hand moving from one place to another (airport).
2. No running off. It is wise to consider holding hands at times in busy areas of the parks and post-parades/fireworks/big shows.
3. Endurance. Say, if one of you needs a nap, sending one of you all the way back to the hotel alone isn't really an option. The better you work as team, the more everyone gets to do.
Maybe something like the 10 year old helps the 6 year old get dressed while you shower, as opposed to playing video games and leaving you to do all the work. Maybe consider options like taking turns with the bag/backpack. Or maybe it is the 6year old's job to hold the hotel room door when entering/exiting with luggage. Little helps like that really add up.
4. bathroom breaks. Our rule is: if one goes, more or less, everyone tries. (Or at the very least, uses the time constructively.)
5. There is always a small chance that a ride will stop. If a guest does something dumb, like trying to get out of a moving car, then the staff has to stop the ride for their own safety. Rarely malfunctions can happen. Most rides stops are very short, but very rarely, rides do get evacuated. Kids should know what to do if they happen.