If you want the motion sensor, check out the X10 products. They make
a small wireless motion sensor that transmits a signal that is received
by a tranasceiver that plugs into any wall outlet. The transceiver
picks up the wireless signal and then sends an X!0 signal down the
powerline. That is compatible with a whole list of devices,
everything from simple plug in modules that can turn lights/appiances
on/off, to X10 wall switchs that you can place anywhere you want within
the house.

But, I have to agree with many of the comments already made. First, a
motion sensor is very different from a fence. I don't think it's
unreasonable for parents with a toddler to want a fenced in backyard.
And I don't think kidnapping is the main issue. The main problem with
toddlers is they can wander off if a parent is momentarily distracted.
A parent could be in the backyard with the child, get distracted, and
the kid could wander off. A motion sensor isn't going to easily or
effectively solve that problem, but a fence will greatly reduce the
chance of that happening. I doubt you will get anywhere in convincing
the neighbor to use a motion sensor.

Another issue is the condo documents. What do they say about fences?
If they say they are allowed, not much you can do about it. If they
say they are, but need some type of approval, ie neighbors, board, etc,
then you can object and may prevail. Or they may prohibit them
altogether, which can be the deciding factor, even if others have put
up fences that have gone unchallenged.