I'm going to disagree a bit with the others... while that boat might work for tubing now, while the kids are little, it will struggle more as the kids get a bit bigger. To be able to sling the tube outside the wake, which is the fun part, you usually need to be able to stay on plane at about 18-20 mph with enough extra power to give it some throttle as you weave back and forth. For older kids, especially if you have a tube capable of carrying two kids, I fear that boat will struggle. You probably have a couple years before you really have to worry about them outgrowing it, though.
Also, use a bridle that clips to the transom for pulling the tube. If you use a ski pylon, you're liable to rip it (and whatever it's attached to) out of the boat when the tube submarines.
When our kids were very little (7-9ish) I would pull them behind our 14' Sea Nymph with a 10hp tiller-steered outboard. We couldn't go very fast, but it was novel and fun for them. (In retrospect, there were less tubing than they were simply being towed around slowly on raft), but it didn't take long for them to want a bit more fun and excitement.