In the UK, associations belonging to the British Aikido Board have to put their instructors through a kids coaching course if they want to teach juniors. This is aimed at giving the instructors a set of best practice guidelines. One of the things we are told is not to do wrist locks on young bodies, this is because the bones are very soft until they reach mid teens, and easy to cause damage, particularly for future development if these techniques are applied too hard. I don't know where the medical evidence comes from, but, I personally believe it to be true. You only have to take hold of a 9 year old wrist that 's about an inch in diameter and you can see how easy it would be to damage if a nikkyo is applied too hard. I don't know about everyone else, but I've been running a kids class for three years, ages 7 - 15, when they train, especially in the first couple of years, they will always try and apply techniques as hard as they can, they have very little in the way of control. For this reason alone I think it unwise to let them practice nikkyo, sankyo, kotegaeshi and hiji shime. I do let them practice yonkyo occasionally though
Whatever medical evidence is out there, pro or anti, I personally wouldn't consider doing these particular techniques on anyone under 14 or 15 years old.