S.,
I have been to neighborhood block Halloween festivals and they were a big hit. They started right after school and were done by 6 at the latest. Last year, the kids stayed together and went trick or treating as a group afterwards.
This year it's a "come and go" as you like from 3-6 and trick or treat on your own.
You are so generous to want to plan this, but as far as "open to the public" in your house, I have to say that's a big risk, especially with children in your home. Our family invites neighbors, school acquaintances, and "friends of friends" to our home, but never a public invitation.
Our Church is in a very safe neighborhood, yet in the last six months, we have had numerous breakins DURING services or activities. Whoever this is finds out when the Church will be open for an event and has slipped into the library to steal and pawn media equipment or broke into the Bishop's office looking for money. It still amazes me they didn't get caught because we have a small, tight knit congregation. Most people would immediately recognize a stranger, approach them and introduce themselves.
We know it's not a member of our Church because it's well known to the congregation that all donations are taken to the bank immediately after Sunday service and most people pay by check anyway. It's gotten so bad, we have to have security standing at all the doors during an activity and all children (including teens) need to be in the presence of an adult at all times, even to get a drink of water in the halls. Unless you live in a town of 500 people and you know them all, I wouldn't do it.
A good compromise would to be to invite friends and neighbors, the families of school classmates and your students at your gym and studio. The studio might be a better place to hold the event than your home.
Good luck,
S.