it's evidence and proof of intent - it's not hard to understand
The law operates on evidence - not gut instinct or moral outrage
The parents in that case (I don't know all the facts) may not have realised she was going to die and I believe have learning needs themselves - that's for the law to decide - hence the charge (not enough evidence for murder/manslaughter)
Boys putting fireworks through a letter box - I'm guessing did not do so intending to kill someone - hence no evidence to support a murder charge
Woman pushes someone in front of fast moving traffic - did not intend her to die - so not murder - but her actions lead to her death - hence manslaughter
Thankfully we have a legal system not based on outrage