I'd like to look at this as a tragedy rather than pinning the blame on someone.
The mother was literally allowing her children to play in front of their house, which is the bare minimum of freedom most parents ideally should allow their kids to have. It's not their fault that a heavily-tinted car was left unlocked nearby. Hindi rin nila fault that they weren't aware that these types of deaths (kids suffocating in a child-locked vehicle) are fairly common and so spent hours passing by the car without looking. And normal din for her to blame the driver out of grief, although of course mali siya and has no case.
On the other hand, driver was only human and forgot to lock his car door.