My child can't stop even when told "stop" or "no." How can I teach this?

Not stopping at "stop" can be because—even when heard—it's too fun to switch off, or they don't know how to stop. It becomes easier to learn if you (1) unify the signal (say "stop" plus a palm-out hand sign), (2) show the next action right away once they stop ("let's do this instead"), and (3) praise stopping specifically. Tell the other child, "when the signal comes, an adult will help too," and watch over them rather than leaving them alone. If it's hard to stop when excited, preparing calm-down time or a place before the activity works as prevention.