OVERLOAD is commonly seen in children as they might be swimming in the morning, running around in school and then have soccer and football training that night.
Therefore they need to spread out their training and take their rest days too to prevent injury!
Growing pains is another thing we see in this category. If they are getting growing pains, adequate rest would also be recommended
Lastly, young people from age 14 up are starting to go to the gym these days, if this is the case definitely seek professional advice on technique!