They have many bikes with training wheels and even have your choice of blue or pink. They also haves trikes as well.
As far as booking the bikes ahead of time, you really do not have to. You can walk up to the bike rental place the day you are there and charge them to your cabin just like everything else. If you don't have them booked ahead of time then you have the option of not using the bikes say for example it is raining all day, exceptionally hot, or maybe you just don't feel like riding bikes that day. As far as I know, there are no refunds once you have pre-purchased the bike rental.