I would be concerned if any junior helmet hadn't been tested against senior weight balls. Given that juniors can play senior cricket from the age of 13 - or younger if they play county age group cricket - and wouldn't be able to wear a senior lid as it would be too large, any brand churning out junior helmets that aren't up to being used in adult cricket would be putting players at risk.
I use the smallest size mens helmet and has never thought to check the overlap with junior sizes. The only issue for me seems to be that the junior lids come with steel grills only