They are in jail because their actions KILLED A BABY.
have you seen a baby that small before?
this is a baby that weighs 3 lbs and some change
and your argument doesnt matter anyway, ignorance isnt an excuse (even though I highly doubt they didnt know that babies need to be examined by doctors, especially one that small).
In your world, if I get pulled over for drunk driving I can simply say "officer, i had no idea it was illegal to drive drunk!", and they should let me go.
"I thought my wife needed to be beaten up, I didnt know any better", and no one goes to jail.
it can go on and on and on. Its not an excuse. Wether or not they knew doesnt change the fact that there is a dead baby, and its their fault.