It's tempting but too simplistic to look at it this way.
Firstly you'd have to prove they were negligent - and before you say it, the incident itself is not proof.
Then if the parents are jailed, the state has to fund care for a number of children, some of whom may have additional needs, and we must consider the additional trauma that would inflict on the children.
And then there's the awful possibility that no-one has considered in this thread - was it self-inflicted.
Ultimately what we have here is a family who will already be going through the worst possible hell. They have lost a child. That is worse than any jail time you could mandate.
The pros and cons of trying to criminalise a dead child / a child with additional needs / a child below the age of criminal responsibility and / or the parents of several traumatised children need to be weighed up. I doubt it would be in the public interest.