Our local athletic league teams are divided into 4/5. 6/7, etc. Until they turn 8, the rules of our league require all children to play all positions. I currently coach 4/5 girls, and we play 6 on, including the goalie. Last season I had 9 girls, so I usually had 3 sitting out each quarter. We play 4 8-minute quarters. I created a system where I rotated each girl from offense to defense to goalie to sitting out. Each game, (if all nine showed) I would have to have 2-3 girls sit out twice. I made sure I rotated that as well each game so the same girls weren't out twice every game. I did make sure my stronger players were in goal and on offense the last quarter, but all girls played all positions. I had a chart, or 'game plan' where it was all written down before each game. Then I informed the parents and players at the warm up who was doing what, when. Usually my girls called out early enough so I could make my plan, but I always wrote it in pencil, so I could change it at the game if need be.
Since they are young, obviously the lines between offense and defense are blurred. Pretty much the only one who stayed put was the goalie

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The main thing was communicating with the parents that this was the rule of the league, not just my whimsy. Also letting them know ahead of time who was playing in each quarter made them more aware that each child was being given equal playing time.
Another rule in our league: technically, we don't keep score until they turn 8. So no one team is 'better' than the other, everyone just has fun and learns!!