I still don't feel like my question has been answered.... is there anything behaviorally or psychologically detrimental to picking up/carrying a young toddler (20 mo) who is asking to be picked up/carried?

Not necessarily: However, you wouldn't want the child to become dependent on others to carry him/her about all the time or to need to constantly be held. I think it is a matter of balance. The child needs some autonomy but picking up/ carrying at times might be just the right thing to do. Be well.

Carrying toddler: No. The toddler is a baby and asking for comfort and a sign of security in a big world . By picking them up when they ask you are showing them that their voice counts which increases their sense of efficacy - I can do, their confidence and their trust in the world. If they complain their legs hurt get it checked out by peds.