Have the cap in adult size (ADLT) ordered and received. It is elastic and should thus fit well on normal sized heads, which she does with me around. Thin and smooth from the outside, you can also get the bike helmet easily set it. The Windstopper inserts and soft fleece-like material, it is nice and warm - as it should be. If so pack up small that it does not interfere in the jersey pocket. The Windstopper however makes them over his ears a little stiffer than the top of the head, so that they are not carrying weightless. In addition, the Windstopper provides some noise attenuation - I think you heard enough, you should just know.
