T-shirt exchange

I bought school t-shirt for my son and it was not the right size at Woolworth Mega City, when I went there to change it for the right size they refused they were so rude to me they said it was dirty, where as it was a slightly mark where I did not even notice. How was I supposed to know the right size without fitting the child? Worse of all they were very rude to me, I was not pleased at all about thier behavior. I trusted Woolworth store for so long.