Here's one that may be an option:
I use their snow tubes and have blast with them with the kids. Haven't tried their river tubes, got a sweet deal on some used NRS from the local guide shop. You'll notice that Tubepro runs butyl tubes inside with short valve stems vs the PVC my NRS uses. I've seen on their site somewhere some neat little party rafts also.
You're right though, the kids love using tubes on flat water floats. It's entertaining seeing how far out they can jump to the tube from raft.