""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 simply cannot expect to send someone a save the date card, fail to invite them to your wedding, and then request 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. However, I am frankly astonished that you cannot appreciate just how rude you have been, in asking me to help you decorate the hall for a wedding you chose not to invite me to, for the benefit of those who you DO regard well enough to invite!
It is embarrassing for both of us, so you'll have to accept a firm no.
Tidy
A few tweaks there.