Leap Frogs is the nineteenth mission in WorldBuilder.
Immediately go to the bottom of the screen, select the Frog, and start grabbing the Frog plans scattered across the map, along with the Treebot plans on the south end of the north island. Be wary of the Alligators, though, as you only have two chances to get at least four frogs to the pile of blue bricks on the north island. You could also try taking apart and rebuilding Frogs that have been disassembled by Alligators to move them across, although you could potentially waste your Frog plans by doing this.
Once you've gotten at least four Frogs across the river, use their parts to build a Treebot for the goal.
Grab the Duck plans on the north end of the north island if you haven't already, then maneuver it carefully across the river to the goal. You only get 3 Duck plans, so the take apart/rebuild strategy won't work as well. Be extra careful.