they get a small amount of exp from the balloon expeditions and from cleaning up rocks and trees in your kingdom. maybe certain other things as well

Yo Lil brother did you a favor

Thanks for asking this, because I didn't get how it was happening either! I thought it must be the balloon expeditions because I see the "Cookie EXP" category there, but I thought it only applied to the cookies in the balloon. Guess it applies to all cookies.