The length of sentence may be related to how the charges were filed. Some jurisidictions file strictly on the top count (the death of the baby) while other jurisdictions may file multiple counts (child abuse counts, assault/battery counts, multiple options of homicide), and how many counts he was convicted of. If there was a legislative change that affected his sentence, I would guess that probably he was convicted of multiple offenses based on this one act?
As for him being around kids: A 21 year old left with a 2 year old and a newborn has a much lower frustration threshold than a 37 year old. And I'd also be willing to bet money that he didn't have much, if any, any parenting support. I would be willing, if the investigation doesn't turn up any major discrepancies in his story, to believe that it was an accident. There's not much more fragile than a newborn. I've seen cases of "shaken baby" in medical records that occurred because of rough play by older siblings.
That said, even if it was an accident, it was born out of his frustration.
He may have learned to control his frustraton levels, and maybe learned how to handle himself around kids. That said, your cousin needs to know that she needs to take this VERY, very slowly, and not just because his conviction is for hurting a child. This guy has been in PRISON for nearly half his life! There's a lot of institutional mindset that he's going to need to overcome before he's remotely a candidate for anything other than casual dating.
She shouldn't even introduce him to her kids for AT LEAST a year. She needs to build a reliable relationship with him, entirely exclusive of her children before she allows her children to potentially get attached.
If after at least a year, they're still dating and considering taking the relationship beyond dating, then he STILL shouldn't be left alone with the kids for a least another year--because no matter how long ago it was, or how accidental, his daughter is still dead at his hands. Before he's left with the kids, he needs to have a solid track record of supervised interaction.