KHS T-Rex is a perfect transition bike for kids that want to take the next step and get a multiple gear bike.  
It has a low, sturdy aluminum top tube for easy mounts and dismounts, and lightweight aluminum wheels for easy pedaling.
Plus, the 21-speed drive-train shifts easily and makes climbing a breeze. Also, it comes with a suspension fork that keeps the ride nice and smooth.
The 24" wheels perfect for the older kid range up to 4' 10" tall.
Pictured with a front fender that is easily removable.