I do think it's basic human decency to make up a lie that won't be rumbled!
It's so disrespectful. But it's amazing how badly some people will behave if they don't have the guts to handle a tricky or uncomfortable situation.
If she didn't or couldn't take your children for whatever reason, the mature and respectful thing to have done was to tell you in a firm yet polite way. Failing that then she should have made up a face-saving lie that would have accomplished the same thing. Lying in a way that you then find out and are left hurt and upset and confused is just not a nice thing to do.
And sometimes that person doesn't mean to cause hurt and is just crap at those kind of social skills, and I have sympathy for that. We're all human after all, and people can get very anxious about stuff like this, and then just make a mess of it in the worst way possible in some kind of car crash slo-mo 'noooooooooo' kind of moment, and feel terrible after. That's just shite for all involved and maybe you should assume this is what happened rather than anything more unpleasant than that.
But others do it because of a kind of cowardice & laziness, ugh!! No sympathy or time for that at all!
It makes me cross because it's all such a needless way to behave, to upset someone because you didn't value that person enough to deal with it properly, or rather, that that person values easing their own uncomfortableness above the other persons hurt feelings.
It's a bug bear of mine, in case you hadn't realised 
Try not to take it personally, which I know is so hard to do when she seems to have been pretty damn personal.
If she really has made up a ridiculously transparent lie to avoid dealing with you honestly, and now doesn't care that she's causing you upset as you discover her lie, then she's not a great person to be friends with anyway. And if she's behaves in such an inept way you could also try and feel sorry for her that she doesn't have the skills to handle situations where she needs to give news in a sensitive but clear way. Sort of a fellow feeling type of sympathy?
And if she wasn't lying at all, as other less cynical and suspicious posters have said, then I'll happily eat my hat!
🎩