Are there any tips on keeping a 6 year old on the sleeping mat?

We tried our new family tent (the Tarptent Hogback) out in the backyard last night. While it worked great, our 6 year old son moves a lot. He ended up in the morning on the tent floor with his sleeping bag (Eureka Grasshopper) and the mat (Therm-a-Rest Ridge Rest short) was nowhere under him.
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Please feel free to disregard my opinion.
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