Here are a few suggestions:
First of all, there is risk involved in all we do. We have to accept that the children could get hurt wherever we are. Having said that, we ought to do all we can to minimize the risk. Review basic rules/boundaries for their behavior dependent on their age and capabilities (climbing, pushing, jumping, running, etc).
1. I would think I would stay right with any children age 3 and under or give them strict boundaries (which apparatus they may play on). For older children...
2. Try walkie-talkies that clip onto their belts/waistbands so you can communicate.
3. Bring a whistle for pre-arranged signal that it's time to report back to a designated spot (wouldn't work for very young children who lose their bearings.
4. Have a buddy system with siblings. They have to stay with a sibling, "or else..."
5. Maybe even have chiming watches for the kids that are set to go off when you want them to report back to you at meeting spot. Have the kids check in every 5 minutes or so.