As a parent, I know there have been times where I am in a store and have simply said " no" to a toy to my son and had two hours of screaming afterwards.
It is not because I am a bad parent and can't say "no" to my son but because he gets fixated on something and in his mind that item is what his whole world becomes about.
I have had someone say " You need to discipline him more! He's so rude." Well, that isn't true.
My son, like many other children with Autism have trigger items and words. By this I mean items he becomes fixated on and HAS TO HAVE or words he responds badly to. One of those words is " no". He has always turned violent and becomes very unrelenting in his screams and insistence on buying the item( if its that, sometimes its sharing the T.V with Mom.)
One thing I learned to do is carry a card detailing what Autism is and how it could effect my son.
There are some that are free and can be printed and cut out at home from card stocks, while there are some made by foundations that specialize in Autism disorders.
Free to print:
Reasonably priced cards:
1) $5 for 50 cards
2) $10 for 100 cards.
I will add more sites to this list as I come across them both for free templates and for cheap mail ones.
I hope this helps.