The buckle is absolutely the key thing.
As
@Bonefish Blues mentions there is a safety angle - I suspect he and I are thinking of the same tragic case (Upper Heyford, not far from here).
But the other thing is that it is
very easy to accidentally catch a wriggling toddler's skin in the buckle clip, which results in a whole lot of crying and an "I don't want to go on my bike again". To avoid this you end up having the clip loose... which defeats much of the point of the helmet.
There are some good clip designs, and you can also get magnetic fasteners.