Do you know of any judges in your region? We're having a mock meet in a couple weeks. There's a judge in our region that's coming in to do it. There is a fee to have her come in so we'll all have to pitch in like $15-$20 for the meet. We also have a coach on staff that's a judge and she may also be judging, I'm not sure.
I think one of the primary things to work on is covering your staffing issues if the coaches work with more than one level or group. Our level 6 coaches are also our level 7 coaches. The 7's had a mock meet last weekend from 8-12 and our practices are usually held at that time. So, we had to come in at 11 instead.
You also need to think about use of the equipment, how is your gym set up now? How will a mock meet take away from those that are practicing? Do you want them all in a team leotard or doesn't it matter? There are lots of little things to work out, even for a small scale meet like you're thinking about.
Or, make it into a meet where you would cancel all practices, and have your coaches be the judges. Our gym has a Class Meet every spring where the developmental girls and lower have a mini-competition. They are broken out by age group, like a real meet, move from one aparatus to another, usually led by an older team gymnast. The team coaches are the judges. Everyone that participates gets a medal or a ribbon. My dd actually won the class meet about 3 years ago. It was too cute. She still says that's her favorite trophy!
Good luck!