On our first trip in 97 our boys were 10 & 11 and we let them go to the arcade and the pool, and the football field at All-Star Sports by themselves.
On a trip in 99 when they were 12 & 13 we were on a trip with a group of 10 (6 kids (ages 11-16) and 4 adults), the 6 kids would go to a different part of the park than us or go to the pool, food court and arcade by themselves. There were a couple of times we even let them hop on a bus back to the resort and go to the pool or arcade while we adults had a nice dinner. We also let them go to Disney Quest while we spent the evening at Pleasure Island.
On our last trip in 2002, our boys were 15 & 16 and we let them have free reign of all of WDW with charging priveledges. We only made them check in with us for our one nice meal of each day, and of course, they had to be back at the room on the first bus after Disney Quest closed or call us to meet them at PI's gate so they could join us in Adventurers Club.
This was with the knowledge that if they abused either their free time by not meeting us, or charged above their budget, they would have to spend the rest of the trip with us.
We have cell phones with a family share plan, that allows us unlimited calls between each other with no roaming, so that let us stay in touch. If we got their voice mail due to no reception or if we had it off during an attraction, we didn't worry too much unless they didn't call back within an hour or so.
What was really nice on the last trip, is the boys chose to spend a couple of mornings for EE with me at the parks while their daddy and uncle slept in
The only thing I regret about splitting up so much is that I collect the attraction photos, and we never managed to get any with all 5 of us on them.