Set up a large trampoline at the gym or in the grass at your school. Encourage kids jump and tumble on the trampoline, one at a time. Demonstrate how to complete a somersault on the equipment and see how the kids do. Encourage them to move slowly and watch closely, so the kids don't accidentally roll off. Having a net around the trampoline can help.
Show the children how to roll a ball backwards and forwards between their friend for a while. Then demonstrate how to roll their bodies forwards and then backwards on the grass or other soft, padded area. Encourage them to keep their back straight and rock forward and then back without hitting their head on the ground. Help them roll slowly so they don't smack down on the ground.
Encourage the kids to walk around the like animals. Get on all fours and show the kids how to walk around like a bear with their legs bent. They can also walk like camels like keeping their legs straight while walking on all fours. Teach them how to do bunny hops, rolls in the grass and other animal walks.
Say "go" and encourage the kids to run outside in the grass. Then say "stop and tumble" to let the kids know that it's time to stop and roll around on the grass. Show the kids how to roll by doing a somersault or laying flat on their backs and rolling on their side. Explain the rules before going outside. The kids might like listening to music while they perform these activities. You can also combine this activity with simple obstacle courses that involve using a jump rope, skipping and then running and tumbling.