Erin,

If a bike shop can't fix it, a lawn mower, garden supply or small engine repair place might be able to. Rototillers and lawn mowers use the same sort of hand grip, cable-type of mechanism.

Children fiddle around with things...its in their nature to explore their environment through touch. They don't always appreciate the difference between expensive equipment and toys; especially when they are young. Developing that appreciation comes from good parenting; something that is sadly lacking all too often.
The parent should have stepped up to the plate when it came to light that their child had messed with and may have damaged your walker, but as they say...the apple doesn't fall far from the tree.

I hope that you find a repair shop that can fix it without much expense. It sounds like replacing the cable mechanism should do the trick.

With love, Erika