I would strongly recommend against taking a small child to Haunt. The atmosphere is designed to be creepy. The props are gory and disturbing. The paths are chaotic and threatening. Even with the No Boo necklace, the actors are still in plan sight, and they wield bloody props and look terrifying. I know the scare zones would have given me nightmares as a child.
Additionally, I would suggest that while the No Boo lanyard is a possible solution for the individual, it affects everyone else's experience, as well. The actors have to avoid frightening the person with the necklace, which means that everyone in the vicinity misses the opportunity for the scare, as well. Since people pay a lot of money to experience a terrifying event, I think that except in very special cases, the No Boo lanyards are actually fairly selfish, as they deprive everyone else. Taming down haunt for the individual has broader effects on everyone else, which does not seem fair to me at all.