I have done it with my well above averaged sized 9 year old son and my teeny weeny 7 year old daughter. The biggest problem was finding each other after being separated. Since there was a lot that they could not do together, including being at the same depth in the wave pool, we ended up splitting up. I spent most of my day hunting down one or the other or both. After I collected them for the second time from the lost kid area, I told them to stop going there when they got lost. We picked a new meeting spot and they were only to go there if they were hurt or hopelessly lost.
It was hard but doable but then again, I am the type of mom who never worries about anything. If you have anxiety about losing a child or two, don't let the kids outnumber the adults. You have got to play man to man, a zone defense will never work.