Hello, I used to work there as an actor, for about 3 years.
To those sharing their experiences from their own childhood, I would say it is a completely different attraction now. It was bought by Merlin, who also own Legoland, Tussauds, the London eye etc and it is much more of a family entertainment experience than a gory museum.
That said, it is intended to be spooky and scary, and they make good use of things like disorienting lighting and low level music to put you on edge, combined with sudden effects such as loud noises or squirting water to make you jump.
It is all designed with a roughly 11 year old in mind, and every day we had hundreds of kids through aged 8 and up. I agree with previous posters that it depends on the temperament of your child as to whether they would enjoy it.
The actors are all well trained at judging the crowd. We do not touch people, unless providing first aid (which happened a lot!)
We are trained to look for the people who will scare easily, but in the right way, ie a scream and a laugh, not a scream and tears! If someone in the group looks genuinely freaked we would leave them alone.
If at any point it is too much just hang back until your group is moved to the next room (roughly every 6 minutes) then approach the actor and ask to leave. They will drop out of character, whip a radio out of a hidden bit of the set and call a manager to walk you out through the shortcuts (takes only a minute to get to an exit at any point)
The actors job is to create an atmosphere and make people jump, but they are also there to ensure everyone has a good time, so will help you if needed.