Spiking drinks - dang I've drunk in lots of pubs for lots of years, no one has ever spiked my drink.....
But seriously, nope you don't have proof, you believe it to be true AND more importantly it has NOTHING TO DO with you or your kids.
Ok so leaving them to go next door. Again, not great - but the eldest is 10.... Mate my kids get off the school bus, walk 10 min home and stay in the house for 30 minutes till I get home from work. Not every day, but occasionally.
Ok so you've said they are unsafe.... and I also understand the Ice thing... But what you've got so far is pretty flimsy.
Look at this point I think you're right to worry. But, you really do want to think about the pro's and con's of doing anything...
My advice, keep an eye on things, if the kids school attendance drops off because mum is too hung over to get outa bed and get the kids to school, or if they are coming to you unwashed and complaining about being hungry constantly and telling you things that just don't sound right, then get back on this site...
Now the other thing to consider is how you will manage full time care in the short term.
Look if she is taking ice, let her, none of your business, BTW the stuff you read in the newspapers about the stuff is mostly true, but some folk do manage to use the stuff recreationally without getting addicted (according to a friend who is a drug and alcohol counsellor)
One last thing - The longer she is on ICE the more obvious it becomes she is on it... Again get back to us... Once that is the case then removing the kids and calling Doc's is on the cards. You can then insist through court that she do drug tests before seeing the kids. BUT the court may not be interested, so collecting grounds for concern is your best bet. Not evidence, BTW just grounds for concern because it is up to the courts do determine evidence - not punters like you and me...