Need help with setting up match with another school

I have set up a match with another school chess club that has players from 4th-6th grade.  The other chess club coach and myself are complete novices in setting up a match like this, so I was looking for help/advice. Here are my thoughts, and I wanted to get some feedback on problems with this, or what other methods might work better:

we have about one hour to do the match, and I would really like the kids to have a chance to play two matches each. Each of our chess clubs have about 20 kids, and I would say about 80% to 90% of them are able to finish a match within 30 minutes. My thought is to group the players into groups of four (for example, top two players in each chess club) and play round-robin for two chess matches. Regarding fitting a game in in 30 minutes, the kids are not used to using clocks so that doesn't seem a good option. The only thing I can think of is, whoever has captured the most material (unless there is an imminent checkmate on the board) is declared the winner. Does someone have a better option than that? Also, I'm sure that there will be an unequal number of chess players, so what happens if one team has 19 players and the other has 25 players? Any novel ways to handle that?