I have sympathy for the mother. It's the father who has been in the public eye in the past, and who did the talking at the press conf so obviously he's the one the media has focused on.
If they were involved in a joint enterprise, both can be found guilty. Arrests are usually made to allow extra protection for all concerned. It allows the suspects legal protection and the police the opportunity to search and stop any further communications between suspects and others.
Murder involves intent, intent to cause death or harm. Recklessness used to be used as a lacuna (any other legals out there remember the fire bomb thrown into a house on the route of a march in Northern Ireland that killed three boys, I think that may have led to change re: that lacuna and recklessness etc). But the law has moved on, good thing too. Manslaughter is when an unlawful act causes death, as far as I remember. It's been a while since I practiced criminal law!
As for speculation I agree that we ought to be careful. Each to their own and that but I remember being a twattish little pleb working in an office in London when Matthew Kelly was interviewed regarding an allegation of child sex abuse. I was a mouthy little shit, brand new into the office environment and put forth my view that he had to be guilty, no smoke without fire etc. I was put in my place (more gently than I deserved) by a lovely, level chap I worked with who said let's just see, basically. This came to nothing, he was the victim of an attempted sting, and I learned my lesson.
But, that's not to say others can't speculate to their heart's content. Just remember should you be arrested for a crime you didn't commit how you would like to be discussed.
RIP little ones.