The fact that she hid her daughter's death doesn't mean she killed. Her death can be accidental and she could have been just so ashamed of not having watched her or maybe she feared other people wouldn't have believed she was innocent, or she could be so distraught to totally deny her death until she was cornered and she couldn't deny anymore. Nobody can really know what a mother feels when her children die, except another mother that experienced the same tragedy, but what's for sure denial is a reaction that can happen in both innocent and guilty mothers, you can't prove their guilt with it.
Edit: or, as d21lewis and others suggested, she could be guilty of manslaughter but she feared to be charged with murder.