I would not go back. I would start looking for a new job ASAP. This is a dangerous situation for you, your coworkers, and your company. If the kid dies from underdosing and it's discovered that staff "knew" there will be a huge legal nightmare.
This must be reported immediately to CPS (because the mother is causing harm to the child) and they can do an investigation.
I would also notify in writing your supervisor of your "suspicions" in clear detail and print out a copy of your email. In the email write that you would like to speak in person about this as well. In the email inform him that you are notifying CPS because you are a mandated reporter (even if there are suspicions) and the situation needs to be investigated.
Prepare to lose your job.