Commoners have 4hp. So, for reference, 4 successful baby scratches drops an average, normal person to zero HP and they are rendered unconscious and have about a 40% chance of death if they don't receive medical aid.
But STR/DEX would be 1 bc 0 is not possible. Would have a -5 to hit - so 30% on an AC10 unarmored commoner.
I think this means you have a 0.3% chance of permadeath in an encounter with a baby if they only make 4 attacks and you don't hit back and they don't keep scratching your unconscious body.
If they attack you 10 times (1 minute of scratching w/ 6 sec turns), there is a 20% chance they land 4 scratches on you and you have a 40% chance to permadie once downed - so 8% chance of permadeath in one 10 turn encounter with a baby with the same stipulations as above.