It depends on whether you are doing actual supernatural ghosts, or just a creepy old house. If this is a fantasy genre story, don't hesitate to give the ghosts/zombies/monsters horrific histories and appearances (are there illustrations? If so, I don't recommend to have the characters too ugly - they are capable of putting kids off their food). Like Lovely-Words said (typed), kids hated, hates, and will hate being treated like idiots. A lot of 10-year-olds are already reading Shakespeare. Don't underestimate them. Including real-life companies/brands/cartoons, the book will be more appreciated by children, but don't put any spoilers or too much details about a cartoon because the kids who don't watch it will get annoyed. Use a few longer words, teachers might stop kids from reading books that are too simple. According to my brother that happened to some books by a guy known as Geronimo Stilton (who pictures himself as a rat) and the series were banned for Y5 and above. Try not to include swears, that will make the books very unpopular for parents and teachers. It would be nice to have a few characters with interesting personalities, children are really getting tired of the 'good-nice-kind' sort of character (or so my younger relatives claim). Kids are getting sharper than before, keep that in mind, the modern average primary student is quite different from the 20 years ago primary student.
I'm a little unsure whether you're looking for words the young characters in your story would use, or guidelines on what sort of vocabulary is appropriate for that age range. If you're interested exclusively in characters, listen carefully to how kids really speak, and pick a specific age: a six year old will not talk the same as a ten year old.
If you're also considering changing the vocabulary of the story (like, the narration itself) to be more appropriate for children, my advice would be not to. Don't dumb things down for your audience: plain English should be fine. It's usually possible for kids to work out an occasional unfamiliar word from the context, especially if the story's being read aloud.
I hope at least one of these points helps. It sounds like an interesting group to be a part of: be sure you make the most of it!
I second what =star-blazer said find books for that age range and see what they have written. The thought that you should beokay because kids are simple and use simple words isn't enough. I think for what you are doing, some examples are needed. Writing for kids is not my forte because I just haven't done it. All I know that you might want to avoid a character in the story dying and that is about it. Not all kids are made exactly of the same mind so you want to keep it light, possible funny.
Firstly, ignore ~The-Vibeke; she gives horrible advice.
Secondly, when writing for children, don't throw in random words from the thesaurus just because. While it's good to offer a few words to help widen their vocabulary, don't over do it. Kids aren't as stupid as people like to believe. Don't talk down to them or patronize them; no one wants to be talked to like that, especially kids and they'll know. Oh god, they'll know. And children's stories can be very implicitly dark.
Are there ghosts? Is this paranormal or just the kids are spooked because they heard stories about the house? If you're going the paranormal route, give the ghost horrific backgrounds and deaths. Don't shy away from the topic just because they're kids. Kids don't want to be treated like they're stupid or won't understand. They will understand and love the story all the more for it.
To answer your question, they'd look around. You said they'd play hide-and-go-seek so you answered your own question. They'd probably be insanely interested in what was in the house since they'd probably never seen stuff like that before outside a movie or on television.
Please use age approiate words (no swear words please).
Fuck that. This site is PG13 so I don't see why I should censor myself.
Have you tried looking at some books for that age range? Reading a few might give you an idea of how to word your own work! I'm sure there are some "spooky story" books out there for younger audiences (Scary Stories to Tell in the Dark is a good collection) that you may want to look into.