I don’t think there’s a rule about it. Let’s call it a matter of safety and hygiene.
Assuming a child could slip and fall on wet, muddy untied shoelaces, if they come untied regularly, you can bring this to the attention of the parents to let them know that for future purchases, Velcro might be a better option.
You could also ask parents to double knot laces. However, some kids might get shoes on sale (fewer choices) or hand me downs where the choices are more limited. I don’t blame you for being irritated about it, but I think it would be overstepping your bounds to presume the right to tell parents how to dress their kids.