The Road Not Yet Taken
Looking at this, I can see three possible options for you:
#1. Go with the happy meal message and go with a time on her terms.
#2. Tell her no, that you're busy then, and then choose to re-establish or cancel the plans on your own terms.
#3. Do not respond.
Out of the three, I would go with do not respond. This clearly shows that you are ticked off that she over-scheduled on you, and that you are not willing to tolerate her garbage for any reason whatsoever. She will feel like she wronged you, and then she will respond with an apology and attempt to beg for your forgiveness. There is always the possibility though that she doesn't respond. In that case, just move on.
It is entirely up to you though. Just make the decision that you feel will be best for this situation.
"There is no better than adversity. Every defeat, every heartbreak, every loss, contains its own seed, its own lesson on how to improve your performance the next time." - Malcolm X