Cannabinoids have been shown to affect axon growth in a developing brain. This does not prove its important for a working brain, but it provides evidence that it can certainly affect the brain in some way. I'm a huge fan of smoking pot and I think its great and I'd almost be ready to say fuck it, but would you let your one year old eat a brownie or a vape (cause I know you wouldn't let him/her smoke it)? There may be endocannabinoids in breast milk, but there's no need to risk adding more.

Then again, the effects are probably very minor...