Yes, the royal children would have to wait until the age of 18 to get their own arms (heraldry), but they could wear the coronets as according to rules (i.e., children of the heir apparent to the heir apparent) at ceremonies. Princesses Elizabeth and Margaret wore theirs at their parents' coronation when they both were under the age of 18. Their coronets would be of the heiress presumptive (similar to the coronet of the Prince of Wales, or the heir apparent) and the spare heir to the former but I suppose their coronets were that of children (or at least daughters) of a sovereign. I would need to look closely at their coronets in a picture.
If their uncle King Edward VIII continued to rule as Britain's ruler instead of abdicating the way he did, he would get his own coronation ceremony the following year so I would suppose Elizabeth and Margaret would wear the coronets of children of brothers of the sovereign, wouldn't they? Or maybe just Princess Elizabeth would wear the coronet of the heiress presumptive of a heir apparent while Margaret would wear hers of a daughter of a brother of a sovereign, would they? Their father would be the heir apparent to his brother's throne for a while until his brother's marriage and eventual first child.
--Previous Message-- : : I've no idea Dawn. I have no knowledge of : heraldry though I think George would need to : be 18 before receiving a grant of arms. : :