How comfortable you are on your bike largely depends on the quality of the saddle. Whether you're cycling around town or competing, the width, design and components of your saddle should be carefully considered.
You should choose your saddle based on your use and therefore the position you want to ride in.
Although the material is important (a gel saddle will be more comfortable than a foam one), it's more the position of the saddle that makes the biggest difference.
For Kids Go for a size S saddle. For a 30° position (common in racing) Choose a medium saddle (if it's too wide, it could cause rubbing and make sitting down uncomfortable). A medium saddle and a 30° position distribute your weight between the handlebars and saddle.
For a 90° position (straight back, a position often seen around town) Go for a very wide saddle (XL). This distributes the cyclist's weight over a larger surface area so that the pressure points are less pronounced.
For a 60° position (a halfway position, for hybrid bikes) Choose a wide saddle (L).