So this was her reply to you:
Here it is:
"I'm really sorry, I thought we'd talked about this...
I couldn't invite everyone from the save the date list so I'm sorry you and [DP's name] can't come.
I'd still really appreciate your help with decorations etc. Is there any chance you can do it?
Please?

F"
That's it, names removed to protect the innocent!
And now you need to respond again. Or perhaps ignore her completely and leave her wondering.
Is it worth sending her this link, which includes this quote "Because save the date cards are a relatively new wedding tradition, the rules of etiquette for them are still being written. It is certain, though, that you must make sure every person who gets a save the date card also gets an invitation."
I think you might be as well to be blunt about it now.
I am sorry if I wasn't clear before. I was hoping that you might realise just how rude you are being without me having to spell it out to you.
You cannot send someone a save the date card, then fail to invite them to your wedding and then expect them to arrange childcare and put in time and effort to do you a wedding related favour.
I understand that this is your wedding and you have every right to invite whoever you wish to attend, but I am shocked that you cannot appreciate just how rude you are now being in expecting me to help you organise a wedding you chose not to invite me to, and I am saddened as to what this says about the way you view our friendship.