IMO an "error in judgement" is wearing open toed shoes in a snowstorm, getting a little too tipsy at a party and telling your friend what you really think about her husband or yelling at your boss while at work.
Bringing a convicted felon into your home after knowing him for one month and allowing said felon to watch your child for hours at a time is much more than an error in judgement. If your child ends-up missing and/or dead, this is criminal negligence and should be charged as such.
In these days of online people searches and social media, anyone can search prospective dates/partners and get their backgrounds in less than 15 minutes.
Being a parent is a responsibility. If a girl/woman isn't ready to put her child first and take on ALL the bags and baggage that comes with parenting she should put her child up for adoption.
ETA: I was a single mother until my son was 3.5 years old. This entire post is IMO