This is a very simple game you can play multiple ways.
Hold 2 hula hoops underwater and have your child streamline through them. Hold them close enough to the wall so the swimmer can get through both by pushing off the wall.
Give clear goals that are “difficult” but easy to accomplish. For example, the first time they go through the hoops do it normally in streamline on their stomach. The second time, have them go through on their side, then on the back. Each new “goal,” / challenge will increase the fun and sense of accomplishment for your swimmer. By changing how they go through the hoops, you’ll prolong the relatively simple nature of the “game.”