I run a kids AikIdo class in North Wales, England and have asked the same question to the head of our association (Sensei Alan Ruddock who trained with O'Sensei).
Although not based on medical fact, he was of the opinion that kids are able to do most things but he said he would not let kids do joint techniques eg Koto Gaeshi, Nikkyo etc.
I think this is because the joints and bones of children are still forming and any joint techniques if applied to severely may damage the childrens joints or bones which may lead to problems in later life.
Having taught the kids class for about a year now, I tend to agree with him. The kids I teach are in the age range of 7-11 and they don't seem to quite grasp when they are applying a joint technique to severely and after consoling a number of crying children I have decided to leave these techniques out of the lesson until they are old enough.
Even so, even after all the crying I have found the kids to be pretty resiliant to most things but as a precaution I have decided not to teach the more painful techniques..
Hope this is of use.