Six minutes does sound short until you run it. In 3v3 there is very little hiding and the game can be constant.
I’d rather have several short games with quick changeovers than one long game where the intensity drops and the same matchup carries on for half an hour. The idea is that the children play multiple games across the matchday, not that six minutes is their whole football experience.
Try six or seven minutes and see what the players look like at the end rather than judging it from the clock.
If they are desperate for the next game, that is not a bad problem to have. Get the next teams on quickly and go again.