We went last year with a total group of 12 people - including 4 kids under 7. My absolute biggest concern was losing a kid and not realizing it because everyone assumed someone else had a kid. (And in your case, your mom would probably fall into that category too)
So we were *really really* strict about children hand-offs. Before we left in the morning, each child had a designated adult who assumed responsibility for that kid. And if they wanted to hand off the kid, they had to get positive verbal confirmation from the other adult that they were now in charge of the kid. I know it sounds ridiculous, but it worked. It was quick and simple "I'm going on Space Mountain. Mickey, you're in charge of Pluto" "OK, I got pluto" and sometimes we got super silly about it (BIL singing "check baby check baby 1-2-3-4" was my personal favorite). I'm sure the kids didn't even notice, but it kept everyone accounted for.
Other than that, I agree with the others - don't expect to be together 100% of the time, take lots of pictures, don't worry about doing it all, and just have fun!