I agree that you guys would probably be best sticking all together.
Your older ones are a little young to go it without mom or dad, especially this being their first trip, it is overwhelming.
We take all of ours (7 ages 14, 12, 9, 7, 2, and 12 month twins).
We stay together part of the day, and we do utilize child swap. But, we also grin and bear it and go off with the bigger ones, leaving one of us (taking turns) stuck with the little ones which is difficult since there are three of them. We also utilize the baby care center during this time for the babies to unwind a little. Our last trip at Christmas was the first time we allowed the 14 and 12 year old to venture off together a little bit without us. But, they have been there ALOT and we have been working towards this for a couple of years. Cell phones do make it a little easier to keep up with them
I dont know about the age rule to ride alone though, because we do have our older ones ride splash mountain with the 9 and 7 year olds while we wait underneath in the little play area (which is very nice for little ones).
You will have a great time.
We do have our children make a list with which items they really want to do in each park, each day. We do have a good time, but it does take some planning. Having that many kiddos kind of makes it harder to be so spontaneous.
Hope you enjoy your trip!!