Wee Hiro probably didn't need a child leash. Wee Tadashi on the other hand. He probably had like five. And he'd sometimes slip out of them and run off ANYWAY.
Aunt Cass is secretly glad she didn’t have to go through that phase of raising Hiro and Tadashi. She saw it sometimes when she came over to visit her sister and brother-in-law, and she shudders at the memory of some of Tadashi’s hellion phase moments.
But Hiro never seemed to go through that, and she never really understood why. What she doesn’t know is that Hiro’s never needed a leash because he refuses to let go of Tadashi’s hand whenever they go out. He might be too young to understand the concept of death, but he’s smart enough to know that his mom and dad aren’t coming back, and he’s terrified of Tadashi leaving one day like they did, and never coming back. So Hiro clings like a barnacle, refusing to let Tadashi out of his sight.
And Tadashi, who has similar thoughts even though he’s old enough to know they’re irrational, clings just as hard.
(For the record, Tadashi actually went through six child leashes. Two were the regular kind that were just a harness and leash, one looked like a monkey, one had sharks on it, one looked like an astronaut vest, and the last one was the strongest, most industrial-strength one his parents could find. He managed to get out of ALL of them)