Please don't be an ultrasonic cat scarer. As far as I know, the frequency they output at is within the hearing range for most people with decent ears, so when a young person comes to your house, they will be scared away, ie your daughter.
simple solution is to switch it off when his daughter is outside. the cat wouldn't go near my garden with me in it (when i switch mine off so i dont use up the batteries too much)
34 posts in we have had get a bigger cat, or Tiger. Nobody has suggested a dog.
I was going to say oranges or lion muck but both have been suggested.