Do you think you'll use this all the time? Are the kids that enthused about it? This seems like a lot of trouble and cost and I'd probably just rent when you get there. Try renting every trip and see how the kids take to it continuously. It might be a novelty at first but they'd soon get tired of it....maybe not.
We've watched small kids like yours have a ball if they each have their own little kayak to play with. Of course, it would be better if the lake was small so they could be rescued! I wouldn't turn them loose on a river.