I started my little guy on a Hyperlite 52" board with older Techno Grip boots. He soon transitioned to my O'Brien 56" board because of the easy in and out of the new boots with the draw tight string. You might check out Play It Again sporting goods store for a great deal on used. This time of the season they may not have much. You may need two boards depending on their sizes. To adjust the boots to the board have them stand facing you,arms at the sides,feet the same distance apart as the shoulders. Jump up and land naturally. Set the board down in front of them and center. Place the boots on the board in the same width and foot position as their feet. Attach the boots in this position. Depending on the diferrence in their sizes you will have to make this change in adjustment each time they take turns behind the boat.After a while making this change will get old and you'll need a second board. Hope this helps.