This can be a lot of fun, let your imagination run wild.
Start by building your pontoons. Use about five or six or more for each one according to your taste. Glue them together in a stack until you have two equal "pontoons". This can be a raft, sail boat or pontoon boat, whatever your imagination sees.
Glue sticks across the two pontoons to make a deck. You can use them as they are or you can cut the ends off. If you don't cut them off let the ends overhang on each side so the boat doesn't look like it's square. Use however many you think looks good. Most pontoon boats have the pontoons sticking out in the front and back. If you want a raft let them run all the way to the ends.
Now depending of the age of the children playing with it you can get fancy. A drinking straw can be used for a mast and a piece of paper for a sail. If its going in the water and the children are older you could use another popsicle stick or a chopstick for the mast. Cut cloth or paper for the sail and glue it on. Colored thread makes nice looking guy lines for the mast. Have fun sailing!
Don't forget to use water proof glue and paint!
Keep it safe, no small or sharp parts for "little ones"! Really keep an eye on children playing in or around water!