Yesterday I was at the Primark store in my city in the north of Spain.
I looked at the shelves that had children's underwear, all at great prices.
In boxerbriefs they had pieces until 14-15 years old and there were a large number of garments with a variety of styles, prints and colors.
But there were only briefs up to the ages of 5-6 years in bags with 8 or 10 pieces and with the most childish drawings.
Thus it will be impossible to popularize the use of briefs in kids who reach adolescence, because their mothers will not be able to buy them, even in the remote case that they think of doing so.