Every four minutes would probably be standard.
Whatever you do, make sure you buddy up with all the other coaches so you can vote for each other's kids to make all stars. No reason for the kids who truly deserve it to have an unfair advantage.
just have them play a half-court game, or some modified version. that'll preserve their energy a bit.
With only one sub, make sure you use and spread out your time-outs to give the kids a few rests.
Do the other teams have more than 6 players? If not would it possible to switch to 6-minute quarters? Assuming that's not an option, it may be best to make a substitution every 2 or 3 minutes.
The other teams all have 7 players. One girl was out of town for our first game so we only had 5. Early in the 3rd quarter one of my players got hit in the head with the ball and ran off to her moms lap. Luckily some kids for the next game were there early so one of them helped us finish the game.
How old are they?
I coached 8/9 yr olds for a couple of seasons. We had 7 and played five 8-minute "quarters" one year and then four, ten-minute quarters the next. The 8-min periods are much better. Use all your time outs to give the girls a breather. One game when each of us only had 5 players, I conspired with the other coach to alternate timeouts.
And don't forget, youngsters have a lot more energy than we do. Remember when you were a kid and could play outside for HOURS? Now most of us get winded trying to keep up in 10mins of practice with those Jr. Jazz'ers. They'll be fine.
Oh yeah, don't forget to coach like Jerry and Quin: yell a lot and use profanities, question their manhood...I mean femalehood.