Now they have a body the post mortem will be crucial. It will answer whether it was murder, neglect or an accident.
The youngster had brothers and sisters who have not been harmed so it doesn't fit the killing to hurt the other marriage partner scenario. And the rest of the children are OK so that doesn't fit the possible neglect angle.
Which leaves an accidental death. Possibly the boy died of natural causes or a tragic accident, and the mother concealed it to keep custody of the other children?
We will have to wait to see what the charge is, and the eventual trial for more answers.
As an aside, she is no asian babe. A heavy duty burka would be an improvement in my opinion.