Kids of all ages can learn to do this for themselves. As proof. Here is a 4 year old girl from Japan tying her belt by herself. ^_^
Practicing karate isn't just about punches and kicks. Things such as proper bowing technique, listening, and being able to tie your own belt are also important aspects of Karate. No matter what it is you do in your karate training, take pride in it and do it with your best effort.