From our personal experience, we have typically allowed a rest time during the afternoon to unwind a bit by taking a dip in the pool, taking a nap, enjoying the play areas at the resort (Port Orleans Riverside has a fun one next to the main pool), or taking advantage of the one of resort activities (arts & crafts, games, arcade etc). Even now as my are kids older, it still helps us to do that and fight off the "grumpies." It is easy to get run-down at WDW, so we want to do our best to avoid that.
Since nap time isn't a necessary part of our day anymore as I imagine it isn't for you either, you may just want to take a "temperature" of everyone's mood and and energy level each day. If everyone is excited and ready to keep going, you may not need to take a break one day; but perhaps the next day you will.
Have a great trip. Having a chance to meet up with 9 others sounds terrific!