I can see how this could have happened, having some understanding doesn't detract from the horror of the situation.
My own story is the death of one of my children from something that, it turned out, was genetic.
When I found out it was genetic I also found out I was pregnant within the same week,my then husband had also ramped up his abuse and I had moved away with my dc to an area where we knew nobody.
I absolutely shut down. Refused to accept I was pregnant, refused to even acknowledge it because then I would have to go for tests and face up to potentially losing another child, my brain just wouldn't allow me to deal with it. I also had no childcare or anything like that and stuck my head in the sand over the whole thing.
I didn't tell a soul until I was 32 weeks, but the longer it went on the harder it became to tell anyone.
(My child was absolutely fine thankfully, but I still guilty every single day).
I can imagine a scenario where she hid it or didn't know, then got to birth, then felt guilty because of her child's disability, then taking a couple of days to get her head around it, then being worried she would get into trouble and so she just fell into this horrific situation, the longer it went on the less she felt able to tell anyone.
Understanding that the Mum could have been in a very difficult situation doesn't mean that the abuse of this child is being excused or accepted.
To things can be true at once the mum being in a fucking tough situation, and the child also being horrifically abused and neglected.
Jail is the right place for her, and I really hope all the kids involved get the support they all need.