maybe something like bok choi, or peas grown for tips/tendrils?

Maybe sorrel?

What about baby greens? I grow so many leafy things in my garden, and harvest them when very young.

Favas,pea greens,chard,leaf lettuce.

Try blanching romaine lettuce, then drizzle it with oil and soy sauce. The texture change is surprisingly good.

I second fava beans. The tips and young leaves can be eaten just like pea shoots. They also stand up well to brief sauteing.

You may want to look into Asian greens. There are LOTS of them.

I also recommend Asian greens, some are mild like lettuce. Tokyo Bekana, Fun Jen to name a few. I love fava beans, but be careful some people are allergic to them.

Asian greens, lettuce and kale

I'll bet she would like tah-tsai. And baby kale is delicious too. Kale is all the rage these days.