Anything from an environment where care is delivered that you can reflect upon and apply to the role of a midwife is appropriate, remember, while the newborn babies are one of the nicest aspects of the role, a lot more of the midwife's role is looking after pregnant women.
Your volunteering at the children's hospital is perfectly appropriate, you'll be interacting with children, their parents and siblings, you'll be seeing the multidisciplinary team at work, you can use all of this and apply it to how midwives interact with patients, families and other members of the team delivering care.
I know other potential midwives try to get volunteering at sure start centres, try to get time shadowing midwives or spend time with health visitors.
In the end, it's how you apply your experience and show an understanding of what midwives do.