@hisman In my experience, it does you no good to lie for your ex. Her addictions aren't your responsibility to manage anymore. You don't owe her anything, let alone excuses on her behalf. Your best course of action with your kids is to be honest with them about the situation, even if they don't understand it, and even if it leads to more questions.
From personal experience and from what I've seen with others who've gone through similar is that this situation isn't likely to get better. Some addicts can sort themselves out, but few do. Most spiral down and hurt a lot of people along the way. It's better to prepare your kids early for the lifetime of disappointment with their mother than to constantly run interference for her.
If she somehow gets control over her addictions, that's great, and this incident can be part of her personal story of hitting rock bottom, but again, you are no longer responsible for her story. Your responsibility ends with your children, full stop.