It is true that you have to book an adult in each room but it's also true that Disney doesn't care where anyone sleeps. They really don't (and this isn't just pertaining to children- adults also can switch the rooms around and sleep how they want and don't have to sleep how it is on "paper" reserved in those rooms)
They CAN'T care or have it against the rules because not only do the stateroom hosts know who is sleeping in the rooms and can easily tell when one is all children and the other is the couple (parents) but also both times we've cruised we've had to go get keys made so that we could get in our rooms! (ie: change one adult to the other room so we'd both have a key to the room we were actually sleeping in, have one chlid's room key changed to the "kids" room as well). So it's very clear they know what we're doing.
Our first cruise our kids were 6, 8, and 12.5- we had connected SPH rooms. Our 2nd cruise we had a cat 6 across the hall from cat 11 and they were ages 14.5, almost 11, 8. It worked the same (IMO) as the connected room. The only difference was a few steps from door to door instead of a connecting door in the middle (which was actually kinda bothersome with both those doors in the middle and at the time we didn't know we could ask for a door stop to keep them open! LOL)
Our next cruise we have booked 2 cat 12 gty's and we haven't got our room assignments yet. I do hope we're close together or atleast on the same deck, but it's possible we may be far apart. However this time our kids would be 15, 11.5 and just barely 2wks from being 9yrs old. I'd allow my 15yr old to babysit at home if we went out- so I can trust her to be there while they sleep. They basically only use their rooms for (barely getting enough) sleep and when forced to clean/change clothes LOL
I probably would not do 2 seperate rooms with the younger two, but for the eldest always being there. Definitely not at age 8/6 and probably not at age 10/8 either. But with the oldest child also in the room it's not a problem.

I think it really depends on the ages of the children and the children themselves too ofcourse- which you'd know them best to know if that's going to work for you or not.
Personally in our family situation it works MUCH better than us paying a fortune extra to have a cat 4 and having our family all crowded in one room. Sure we'd probably do that if we had babies and toddlers and/or all young children- but with older children that would be so crowded for us and just not necessary.
I have no idea if baby monitors would work or how far. I do know I've read that walkie talkies are hit and miss whether they work well or not on the ship- so I'm not sure if that would also be a problem with baby monitors?
I guess for us the age WE'D feel comfortable with OUR children staying in a room alone would be about the age we'd be comfortable with leaving them at home alone (and legally). Which for us is about age 12. No problem with leaving slightly younger children with said child that age though.
