I am a retired youth minister. I have taken many trips to theme parks with kids that range from ages 10-20. I would never let them roam the hotels by themselves. I have taken anywhere from 5-200 kids on a trip. At hotels the adult leaders would take turns on sentry duty.
At the parks I have let them roam around by themselves. (parents had to sign waivers to let them) I would stick an adult with the younger ones if i could, and they tend to stick in a larger group. the 14+ ages tend to go off in 2's or 3's.
I always no matter how large or small the group was, I had some rules beyond the behavior rules. (I have put a kid on a plane to his parents before, but that is another story)
No park hopping, no leaving the park, no going back to the room to get something.
And there is a manditory check in every 3-4 hours at a chosen location. that location is visited by everyone in the group first thing at arrival then we can split up.
At check in I was very clear that each person had to look at me right in the eye for me to check off that they were there. That way no one can say I was there, but you did not see me. (they will try anything. )
If they missed check in and I found them, then they stuck with me the rest of the day. (And they knew I would send them home on a plane, so it never failed.) Also when 2 13 year old boys did not check in, when I found them we went to HOP, CP, CB and Tiki, over and over. After that, and the scuttlebutt that followed that night, every kid was at check in right on time. (I allowed 30 min leeway without telling them, for crowd and ride problems)
the check in was not to be mean or not let them have fun, but for their saftey and they usually figure this out and are ok with it.
What really suprised me was after the 2nd check in, the kids will start hanging around together in larger groups. by the end of the day, we are all watching the parade or fireworks together.
So the morale of the story is,,,, I have done it with up to 200 kids of all levels of maturity, and have never had a problem.
With just two, you could always keep a "distant eye on them"
thats what I did with kids that I had conserns about.