To deal with a similar situation, my son put in fence along one border of his property, put in a gate and locked it. His testy neighbor can no longer access his back yard.
If you are running a licensed childcare facility, then your backyard is probably already fenced. Lock the gate at night or when you aren't home.
In some states, like Arizona for instance, there are encroachment laws. If she put up that fence and it stays up for (I don't remember how many years), then she's given you six inches of land.
How is this lady perceived by other neighbors? Do they have problems getting along with her? Personally, I'd put a stop to her thinking she can come into your yard anytime she feels it.
I had this problem with a neighbor on one occasion. He was a crackhead who stole everything his parents had in their yard and started in on mine. Turns out all I had to do was send him a letter by registered mail telling him he was not welcome on our property or he was subject to arrest for trespassing. That was enough. Never saw him again.
You do understand that if your home based childcare service is not operating within state guidelines that she can shut you down with one phone call.