(Disclaimer = I'm not speaking on behalf of the organization I volunteer with, these are my own thoughts)
Not all parents are careless- there are wonderful parents out there - but then who makes the decision what constitutes a "safe" home with infants versus a nonsafe home? What happens when Family A is allowed to adopt but Family B isn't, and Family B finds out and goes crazy on the rescue? Sure, some families miss out on a great dog - but there are tons of great dogs out there, with other shelters and rescue organizations without such strict guidelines.
As for Dobermans, this breed as a whole is extremely sensitive when it comes to the bond they have with their owners. They aren't a breed that is known for being "flexible and forgiving" when it comes to the amount and quality of contact and interaction they need with their people. Not all breeds are like this, not all breeds need their humans as much as they need oxygen - but this breed, in general, does need their humans wholly and completely. And because of that, it's very important that the homes they go to understand the unique needs of the breed and are prepared to do what is necessary to ensure a lifelong bond.
This is how I feel too - many of the dogs we take in and adopt out are Dachshund and Dachshund mixes. That breed also is very intolerant of small kids - and my question is, why would you want to subject your CHILDREN to that kind of attitude from dogs?
Our "rule of thumb" is no kids under 8 yrs. for larger, "tolerant" breeds, no kids under 10 for untested adult dogs and no kids under 12 for any small (micromini) breed or Dachshund "type" dog (intolerant of kids).
We make exceptions but we tell people the problem we have is we don't have small toddlers to test these dogs with so I'd prefer to be safe than sorry.
A good example of how we do things, we adopted out an adult male Basset hound to some people in Idaho - they had an 8yr. old.
I knew this particular dog had lived with kids - saw his picture with the girls he lived with.
This family emailed me a week or so later, stating the dog had bitten the 8yr. old boy - not once, but twice!
I asked what was going on when the boy got bit??
The boy, the first night the dog had arrived, had gotten up in the dog's face and hugged the dog around the neck.
The dog bit.
The boy did it - again -the next night or whatever, and the dog bit - again!
Um...wouldn't you have warned your child the 1st time that the dog didn't like being hugged??
So by the time they brought the dog back a day or two later, the dog had bitten the boy - AGAIN - for giving the dog a hug!
Now, this dog could easily have been euthanized for the biting behavior - because I placed it in a home with kids assuming it was good with kids!
So with that in mind we prefer all kids be over 12, but have relaxed our guidelines somewhat. It just depends - on the dog mostly, but also the family and their attitudes about dogs and kids' interacting.