How about running the orange stripe, and give the rest a clear poly coat or a stain. Shame to hide the beautiful workworking. Reminds me of the toys my grandpa made for me in his woodshop, that kind of craftsmanship is something you don't appreciate until you are a ways down the tracks of life. That being said, I hope he beats the living crap out of it from miles and miles of play.
PS router some grooves in the 2x4 as tracks...pimpin.