If your kid is the right age to want a bike they probably know what kind they want.
It's a nice idea to give your child a surprise bike on their birthday or Christmas, but if they had a particular kind of bicycle in mind, you might wind up disappointing them when they see it.
My parents involved me from the beginning in the selection of the bike, making sure I knew the basics of what kinds of bikes there were and how they worked and so forth.
They also had me contribute towards it, since it was a large purchase and they wanted me to feel like it was really mine and something I was responsible for.
I adored that bike to pieces when I finally got it. Being involved in the selection was more of a thrill than any surprise could have been.