The kids have to like it to keep practicing. Perhaps try a couple sessions first and if they don’t like it, try something else. It can be a great bonding activity - my husband and I started doing it too and now we do it together as a family. Or you can go to the gym/yoga while they are climbing. You can get a day pass initially and some gyms run seasonal promotions on membership.

Movement really stanks

Yes, and 6 is not too young.

My teen DD does it. She is not athletic and had low muscle tone. Her arms are now pretty toned and sculpted. Very impressive. She's very proud of those muscles. She's been doing it for 2 years.

There are little kids doing it, too. Her coach works with SN kids and said he sees improvement with the kids' self esteem, too.

DD goes to Movement in Rockville.

6 years old is not too young, but the fact that she doesn't want to is a problem.
If you are signing them up with an instructor, the absolute best thing you can do is to trust the instructor and walk away. Do not hover. Why not drop off the 9 year old and take that time to do something special with the 6 year old that they enjoy?

Does indoor rock climbing lesson once a week help low muscle kid to strengthen upper body and lower body? Kids are 9 (low muscle tone) and almost 6. The almost 6 year old is plus one, due to childcare, I will sign her up as well. She has tried rock climbing a year before, and she did not like it and whine about it. Is 6 year old too young? There will be teachers, so I will stand around to watch and pass out water. What can parent do or help when I can't rock climbing? I also tried one year before, and it was hard on me. I prefer watching my kids to make sure they are safe, and I would rather spend money on them than on me. It is not cheap, and it adds up.