It's probably more important how confident they are in deep water. With a wetsuit on (added bouyancy) and salt water (added bouyancy) you really don't need to be a strong swimmer to be able to stay afloat. However if they "panic" and thrash about it will be hard work for them to float and they may get tired. Swimming is 90% floating and that is something most people do adiquately, it's only when your body tightens up it gets hard work.
There is an option of doing the "interaction" part of the swim in the standing depth part of the water and the kids can have a "boomerang" ride out and back holding onto the dolphin's dorsal fin. If they choose this they don't have to go out of their depth. If you choose one of the later swims they can watch earlier groups and decide what they feel they are capable of.
From what you say , I think they would be physically able to go out for the "deep water swim" part of the experience ( it's about 5-8 minutes duration I would guess) , it's more a question if they are confident enough in their abilities to do it.