I had to go and read the whole article after reading your post. She no longer has custody of her son, who was 2 at the time she beat her newborn.
In the 2 yrs since she was declared incompetant she has been in a facility where she has had explained to her so she understands..what she did and what the consequences of those actions will be.
Whether she should have ever had any kids or been allowed to keep them because of her mental retardation is another question, but to state that just because she was able to have babies and care for her mom and an older son means she really understood that hitting an infant in the head would kill the baby, well I have a mentally retarded daughter, she really would have NO idea that she killed a baby, she could be taught to care for someone who is disabled or even an infant, but she does not understand 'death' and that her actions could cause it.
Not all people with MR understand exactly what they have done, in this case the judge said that she did not and sent her to an institution to be taught what she had done. Having said that I'd still wonder if this young woman really does understand what she has pleaded to.
There really ARE people who do NOT understand death is final.