[Icelander]

I'm fine with saying that baby humans are imperfect creatures, not nearly as robust as their weight would suggest. HP 2-3 shortly after birth strikes me as just fine.

We're notoriously fragile for a while, there. A human baby and a full-grown animal that weight the same will not be equally tough, robust and able to survive a machete blow.

Also, when chopping up a baby, you probably ought to be All-Out (Strong) Attacking. That means that an average blow, as noted above, will do 1d+2 cut. Given that you also ought to be Telegraphic Attacking and will have a height advantage, the soft skull is an excellent target. 1d+2 x 4 will most probably do the job in one hit and if not, the baby is unconscious and will inevitably bleed to death anyway.