The main problem is that the jaw hasn’t finished growing in young children. The reason dental implants are so successful in adults is because the implant screw integrates with the jawbone itself, making them incredibly strong and stable. This is the very cause of problems in young children.
Because the implant is so strong, it disrupts the normal growth of the jaw. The tooth stays the same while the jaw moves around it. This can mean other teeth are pushed out, it can lead to spaces between teeth, bite issues, and changes to the shape of the jaw. This results in very unattractive teeth and even a disfigured jawbone.
There is also the likelihood that in especially young children, the implant may not integrate in the mouth. Implants in children between the ages of 5 and 12 are only successful 35.7% of the time. This is thought to be due to smaller jaws and other conditions.