I have so many questions about this, and most of them can just be summed up with why? I mean, seriously: If you can shoot Batman with a ray gun, you can shoot him with an actual gun. That would probably solve your Batman problem. Hell, even if you have to use a crazy mad science ray so that you're getting your money's worth out of having Garth on the team, I can't imagine that a death ray would be any more difficult to build than a machine that turns people into babies.
On the other hand, if they'd done that, we wouldn't have gotten this ...
... so I guess it was probably all worth it.
Back at the hideout, Garth reveals that his plan was to humiliate Batman, rather than turn him into a martyr, which is maybe the most dubious reason that someone could ever have for building a baby laser, but it seems to be working. The Gotham Gazette publishes a photo of Bat-Baby, and sure enough, it's not long before a bunch of pasty dudes in fedoras are standing around talking about how that's the end of his crime-fighting career.
Unfortunately for crime, Garth has royally fucked himself by building a machine that turns Batman into a baby but also leaves him with the mind and full strength of his adult body.