I happened to see someone wearing a 3Blue1Brown t-shirt today and decided to buy one myself to (modestly) support the channel. In addition to shirts, there are also socks and stuffed animals, apparently: https://store.dftba.com/collections/3blue1brown
My son is 4 and saw me watching one of his videos with the colorful Pi characters and was intrigued. After multiple repeated requests to watch "Pi friends" we ended up getting him one of the plush Pi creatures which he still loves.