There's no harm in tricking kids with the placebo effect. The placebo effect is awesome, and "being able to trick kids" is the No. 1 compensation for having them. But you're meant to convince them that a toy has the power to defeat imaginary monsters. The recommended dosage of No More Monsters is three pills per peaceful night's sleep. That's not a placebo, that's addiction training wheels. They're teaching children that pills make the scary things go away. That only works if you're Pac-Man. That kid's going to grow up into a pharmacological pachinko machine.
I've experimented with homeopathy before ...
... and I didn't think anything could make me hate it more. So, congratulations homeopathy, you finally achieved something. Because playing pretend with your kids isn't meant to extend to their medical care. Look at this kid: