I think it would be very difficult to tip over a tube, especially a smaller person. I think the greatest risk is slipping out of the seat or falling forward out of the tube. An O tube with the strap sewn in would make that harder to do. O tubes are not very popular any more because of the difficulty getting in and out of them so manufacturers are making less of them. I haven't used one in a long time. I rarely use U tubes anymore except for hard to get to places. I mainly use pontoons or the raft type kickboats. I think a pontoon would be too easy for a 7 year old to fall out of the seat.

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Joe