Ok, I'm the resident non-believer, making this a non-Christian issue for me. And since I'm hanging around to see if anyone answers my question, I might as well do something while I'm here....
I would look at any of the situations you mentioned as a chance to teach- you do not lie, which you would have to do to say your child is younger and he/she is. You do not steal, which you would be doing by taking something for which you have not paid the proper price. And money does not grow on trees, so sometimes you have to do without something. My kid would have to understand that I'm not breaking a promise- I'm simply unable to keep it RIGHT NOW, but I will keep it, because there's another lesson. You keep your word, even if you must struggle to do it. You may carry a debt for years, but you do repay it, regardless of what it is. And since it's just dinner, I think we could swing it the next week.
Right is right, religion aside. My son's father, on the other hand, is Catholic, and would (and has!) lied in this way. My husband is a lapsed Catholic and does the same, setting an example I am always fighting. I agree with the poster who mentioned humiliating your spouse, but I would do it anyway, just as politely as possible. I could make a joke of his "forgetfulness" of our child's age and later, to make up for it, I'd apologize if it made him uncomfortable but explain that I think the example you set is more important than a few bucks.
Anna