Need help w substitutions....6v6 4th/5th grade rec

Im a volunteer dad coach and I have always had 9 kids so it's easy 3 in & 3 out on every possession change. This season we have 12 kids and I don't have a strategy yet on subs. I don't want to have 6 dedicated to O and 6 on D because I need some of them to play on both sides while still having close to equal playing time. Any suggestions that work for u???