I had no earthly idea there was an Adventure Time MOBA in the works, let alone a beta for one. Lo and behold, Adventure Time Battle Party's MOBA launches today with an open beta, so you can start your kids' MOBA addictions young and all that.
Adventure Time Battle Party features champions like Finn, Jake, Princess Bubblegum, Lumpy Space Princess, Lich King, Ice King, and more. You can register or play as a guest for this browser-based affair, and you can team up for 3 on 3 battles -- if you can find two other people to play with. I attempted to find a game or two to test with, and came up with one, but it seems a little slow-going this time of day.
At any rate, it looks like a nice simple baby's first MOBA, so if you've any interest in Adventure Time or that genre, it might be a fun way to waste a few minutes if you've got a PC and some free time.