You may not put much thought into your purchase when you go out looking for desks for kids, other than perhaps color and overall looks. That can be a mistake. Just as adults can have problems with back, neck, and arm pain from bad form at a desk, children can suffer the same. You want to make sure they have something comfortable as well as ergonomically correct so they don't have aches and pains, and so that they learn good posture while sitting at a desk that they will use in school, college, and on into their careers.
Desks for kids that are very young are often not used as desks. These often become places where they do crafts and perhaps where toys get stacked on occasion. They may even eat lunch there when they have a friend over. Whatever the case, your child under the age of five probably won't sit there for very long. Look at the desk and chair to be sure the child can reach the desk comfortably. Otherwise, you can get what you like for their desk. Just know you will have to replace it once they start school.
Children are bringing home homework as early as first grade these days. That was not something most parents had until they got to fourth or fifth grade while they were in school. This means they are going to start spending a lot of time at their desk. When shopping for desks for kids for this age group, you have think about how much time they are going to spend at the desk, if you need one that supports a computer, and how comfortable the desk is going to be for them. Take them shopping with you so they can sit at each option so you can get the one that you feel will work the best.
Your teenager is going to do all types of things at their desk. These desks for kids are going to be bigger, and have to have more surface space. They should be able to put a desktop computer there, or have room for their laptop, if they have one. They also have to have space for their books and for writing out assignments. This is when you want to make sure they have a keyboard shelf so they can type comfortably, as well as a way to keep a monitor at a comfortable level so that they do not strain neck muscles looking up too far or looking down.
You can look for desks for kids in your local stores, or you can look online. You can not allow your child to sit at a desk you order online, but you can look at the measurements to decide if the desk is what you need. Look for a web site that offers you a great return policy if the desk is not right, or buy from a site that has a local retail store so you can easily return desks for kids that don't work out.