I used to work at Islands of Adventure/City Walk/Volcano Bay when I was still in school. During orientation and HHN training, they would always emphasize that while HHN was 'recommended' for guests ages 13+, there is no age or height requirement for admission so you could, if you so choose, bring an infant into Horror Nights. That being said, I agree with everyone here that it should be on a per kid basis. When I was 6, Texas Chainsaw Massacre was one of (if not the top) my favorite movies. I have coworkers in their 30s now who are too scared to watch horror films. If you know you have a six year old who loves this type of thing, they would probably love to go. By the same token, if your 12 year old is terrified of scary movies, probably not.
Also just briefly skimmed the Trackers top viewed videos.
Of their top 50, the most recent is from HHN 2019. Even after that in the next 10-15 most viewed, one is the pregnancy announcement and only one is featuring the kid. Their channel is definitely declining but they have so much backlogged content that they can still probably make quite the haul for years even with daily "USB and Commode Hunt at Rodeway Inn in Kissimmee" videos.