I too, am deeply saddened by this story, and it hits close to home, my daughter's youngest was just 2 years old when my daughter decided being a mom just wasn't her cup of tea, so I know the hurt your family is experiencing.
I have had the opportunity to watch my granddaughter at preschool just stare at the other kids, engaging with their mom's , and I just see the wheels turning in her little head, I see the look of " oh so that is what its like to have a mom" and it really hurts to have to see these little one's suffer for the mistakes of their very selfish parents.
What a loving father you are, your kids are so very lucky to have you, Bless you, my friend.
Sending you tons and tons of support.