I've been going to haunt events for as long as I can remember. I think it's all up to the kid. The problem arises when they aren't having fun and are being forced to go. That leads to a worse experience for everyone else. If the child is mature enough to go to the park, they should be allowed to.
This might be fine if the only consideration is the effect on the child (and I do not think there should be any argument about babies, in particular, would be scared by the startles, visuals, and sounds), but they are only part of the equation.
This has already been discussed at length in this thread, so I am not going to retread old ground that is easily accessible, That said, at the top level, the presence of children at a haunt degrades everyone's experience.
The actors are put in an impossible position. They are forced to decide whether to scare the child or stop performing. We know of cases where parents have yelled at actors for frightening their children, so there can be retaliation. Moreover, most generally don't want to do that to any kid.
If the actors chose to step back, when children are around, it ruins the experience for everyone else in the vicinity. People who are there for the haunt, want the full experience: sound, light, effects, character interactions, scares, ambiance. In my opinion, it is not reasonable for an adult to pay to attend an adult Halloween event and have that investment wasted, because the presence of children prevented the performers from providing the full experience.