Determining Players for District Tourney

Well, I learned to play chess last year along with my six year old son at his school's chess club. Somehow this led to me being the kindergarten/first grade coach for the school's team. I think we are actually doing pretty well. However, soon I will need to determine who will be going to the district wide chess tournament in the spring (this is the only tournament we participate in). I have seen some info on chess ladders, round robins, and club Elo ratings. After Christmas break I expect to have about 8 chess club meetings to determine our tournament players. I have 12 first grades and 6 kindergartners. The top three in each grade get to attend. Any advice on how best to determine my top players would be greatly appreciated. Thank you!

Who seems to be the best players?