Pretty much what I was thinking - about the finger movement and definitely the calluses! At their age they don't have age-onset arthritis so that's all good. I don't know if they'll be self-conscious. Since they are both learning, they can laugh at themselves together, and if at each other I will intervene. Thanks.

Oh, also, what I am worried about is a fight over who gets the bass and who gets the electric (which I'm assuming is preferred). But since they both admire musicians who play the bass, maybe I have no worries.