I don't have any children myself yet, but I do have many nieces and nephews. In the past, when I have wanted to make a real bang with a gift, I have given them 16 inch bikes, and they can't get enough of riding!
Being able to help teach my nieces and nephews how to ride a bike has been one of the greatest joys of my life so far. Getting them each a 16 inch bike has always been the most heart warming process, and I suggest you do the same for the kids in your life. They love riding their bikes around the neighborhood. They feel like they are fully free!