Halloween Horror Nights is NOTHING like anything Disney does so if you have small children, do not consider bringing them. HHN is a hard ticket event so you would have to purchase a separate ticket for entrance and with the crowds this year, you run the risk of the event selling out unless you purchase ahead of time. There are plenty of threads on the DIS regarding HHN so I suggest you scan those and if you have any specific questions, feel free to ask.
I can't imagine why people rave about MNSSHP...it's just trick or treating...you can do that at home for free. The tickets are waaaay overpriced but it's good for children.
It's not X rated or anything like that. The houses and scareactors get right in your face but are not allowed to touch you and you can't touch them. This year isn't as gory as in past years but it's still pretty intense.
I have a 13 year old grandson who of course is way too cool to react to anything. . . he was screaming his head off at HHN a couple weeks ago. The houses really got to him. Definitely believe "not recommended for children under 13". Little kids will be terrified.