Bok Choy is now one of my new favorite veggies.
Before I discovered it, I thought it was an elite Asian food I would never be able to prepare. Turns out, it's a very common veggie - the bottom half is a mix between celery (without the strings) and a cucumber. It's crunchy and watery and great for a stir-fry.
The top green leaves cook down like spinach and can be added to anything. It's a mild, anti-inflammatory food and shouldn't be repulsive to a 5-year old.
Any vegetable a 5-year old won't fight is a good thing.