For fillings, our vegetarian kids like a bite of pitted umeboshi plum or a slice of pickled cucumber or radish, or some combination.

Taki even grows a small number of ume plums, used to make Japanese plum wine and “umeboshi” salted plums.

But Taki also makes his own umeboshi from apricots and red shiso — which he says have nearly the same flavor as real umeboshi — and sells them at his booth through the summer.

It derives from a pickled stone-fruit snack called see mui in Cantonese, which is itself related to Japanese umeboshi, a snack made of salted ume plums or apricots pickled into sour little balls of delight.

Mr. Miyano’s grandfather, who grew and pickled his own plums in Chiba, shaped onigiri with homemade umeboshi for his grandson’s breakfast.