Making your own broth will be the most nutritious for sure. I like using my Instant Pot, you get an amazing rich broth in an hour or two instead of simmering all day on the stove. Leftover rotisserie chicken bones work great.
Make a bone broth in large quantity and keep it in refrigerator. You can use this as a soup base any time Extremely healthy and nourishing for children. Cost a lot less as well. Delicious and satisfying.
Even the most nutritious broth or stock will be negligible in delivering any nutrients. Try to offer other options when possible and hope the picky phase passes. I wouldn’t stress about making a healthy broth though.
Veggies? What? Boiled veggies by themselves? Eeww yuck.
Throw in some nice bones and some good meat, maybe a carrot and some celery root if you must, and maybe some connective tissues too but try first without those, boil it quickly and than slowly cook it for a few hours and there you go. The meat and bones are what give it the real flavor and nutrients.