This could be a tough one liverpoolfoo, because I think there are possibly two scenarios. 1. Like you said, going out clubbing etc. Or Working nights to feed and clothe her kids. Personally I wouldn't think the worst of this person on what you have heard from the past, as it may not be very fair towards her. Is it not possible for you to speak to this person and tell her your concerns? Or is that completely out of the question? I remember being 14yrs old in the early 90's and my mum leaving myself and my younger brother who was 11yrs a couple of nights a week to work night shift so we could have food, clothes etc. If she hadn't done this then looking back on it, we would have been in much more hardship. My mother done what she had to do to survive. Bet if you spoke with this person she would have a reasonable explanation, and I guarantee you losing her kids is probably the last thing on her mind.
Sorry for being so long winded. But to answer your question, I believed that the legal age for leaving children unattended for a certain duration of time (more importantly over night in this scenario) would have been 16yrs old.
Think you�d be doing the right thing by having a discussion before jumping straight to the authorities, but I think (and I know it won�t be easy for you) the way forward would be to discuss your concerns with her and if she is working nights, then tell her your not happy with the kids being left alone at nights as you fear the inevitable could happen.
I think Daffy & Ethel are jumping in to the deep end without thinking first. Atleast give the woman the chance to explain and (if needed) change her ways.