I used to bring a ball for ds to roll down the slide, because he would want to use woodchips. I would actually bring a few balls, so we could share. I'd get those softball sized foam balls that cost $1 so they fit in my bag easily. I was fine with the woodchips if we were alone but other kids wouldn't like to get their clothes dirty and I would explain that to ds. He was older than your ds, though, probably closer to 20 months. I would encourage ds to drop the ball, then slide down, so the next kid could have a turn. I didn't feel it was a problem to have different rules when there were more kids, like it is ok to go up the slide if there is no one waiting at the top.