It seems that human form takes a certain amount of emotional stability to maintain and can also 'slip' if the user is inexperienced at maintaining it. Examples of this can be seen when Jim Finn is angry and afraid during the 'One eyed bear' arc where he involuntary slips in midform, his arms changing and tail appearing when startled by a car in the The Verona Corn Maize. (See Here)
A second example of this comes from Michelle who can be seen exibiting some traits of her full form in human form, such as sharp teeth and an elongated neck, as if caused by lapse of concentration.
This would explain why children under ten are not given medallions as they would be likely have have trouble keeping in human shape.