My 3 y/o has been drinking pretty much any tea for over a year now. Sencha, gyokuro, darjeeling, green oolongs, roasted oolongs, young sheng, ripe puerh, aged wet stored sheng... Somehow, he likes them all.
I just do very quick brews for him so it's much more diluted than what I drink and has a low caffeine content.
Healthy herbs, rooibos, honeybush, decaf tea, and yerba mate.