Be sure you have purchased Equine Liability insurance for YOUR risk, not just mortality for the horses. Equine liability is not particularly expensive coverage. Unfortunately a location next to a housing development with lots of kids means that you probably have an attractive nuisance. Certainly post signs, but understand that signs alone may not protect you. I would get insurance since it provides legal defense as well as liability coverage.
The laws vary on signage and liability by the state you are in. You might ask your equine liability insurance agent what they require, such as a state law appropriate liability sign. You need to secure the gates also.
Electric fence. A top wire and a bottom wire, plus one across the gate with the kind of voltage that will make the Pope curse. Just make sure the fence is very very clearly marked with warning signs. Someone will get zapped and the whole community will know. And when a parent asks about the fence, just say you're trying to keep deer and raccoons out. No one will ever know your fence is really for people and dogs
Oh and do get insurance and the signs.
What everyone else has said PLUS LOCK your tackroom! If there are windows in the tack room put bars on the inside so even if they break the window, they still can't get in. I've actually got bars on the inside and outside but the outside is to keep the horses from breaking the window as their paddock is just outside the tack room window.
If your fencing is board, put electric about 3" above the top and middle rail. I wouldn't do it at the bottom rail so the horses don't get a foot though (yes it can happen). Then definite signage to indicate hot fences.
Prayer might help also!!!!
I'm not saying let's go kill all the stupid people...I'm just saying let's remove all the warning labels and let the problem sort itself out.