Tough one, but I'd be more comfortable letting them go to the park as opposed to the pool. First, I would guess that the type of crimes that a 12 and 14 year old could fall victim to are far easier in a hotel setting than an amusement park. Other than the workers, everyone in the park paid to get in. That may not be the case at the HRH pool. Second, I don't have the stats on it, but I'd venture to guess that there are more opportunities for trouble or injury at a pool than in the park. Sure they could do something stupid anywhere, but it is a lot tougher to do something stupid on the Hulk ride than it would be at the swimming pool.
I have let me older son (12) and his fried go off on their own at our local amusement park. Not for long periods (must check in every two hours was the rule).
Do you have park tickets for the day you plan to do this? Why not have one of you retrieve them from the park at a specific time so that they aren't wandering around the City Walk unsupervised?