What are the differences

Dentures for children need to allow the jaws and teeth to continue to develop around them. As a result they soon become redundant. Because of this they are usually constructed relatively simply to be flexible, versatile and relatively inexpensive. Children seem to be able to tolerate dentures very well - even if they do not enjoy wearing them.

When you make a denture for a child you have to consider that the teeth are still erupting and the bone is still moving. The implications for this are that the denture will need to be adjusted frequently and possibly made again and again depending on the age of the child. With any denture, if they do not fit well and are not serviced continually, you can accelerate bone and tooth loss. An ill-fitting foreign body in any part of your body can cause trauma and damage surrounding hard and soft tissues. Think of badly fitting shoes, but in a wet environment

Adults have matured and finished growing so the mouth is more dimensionally stable to sit the denture on.  A child is still growing and the teeth still falling out and erupting so the mouth is constantly changing dentally and physically.  This can make dentures in a child more of a challenge but they are not a common thing in a child.

The questioner does not say why the denture is required and the age of the child. There are many alternatives to dentures, from implants to bridges and even children may have an adhesive bridge or a living bridge. Dentures function best if they are well fitting and have a stable foundation of gum and/or teeth to rest on. The main difference is that children are growing and adults are not. This means that a child may require frequent adjustments and/or replacements to the denture. It may therefore be uneconomic to have the very best dentures made for children, only to have to have them replaced some months later.