Unfortunately there's nothing you can do except going along with their request to have no contact with them.
It doesn't matter that you didn't celebrate Christmas this year, they may have found out about the present giving from past years.
Are they still going to be talking to your adult children? You will want to talk to them, show them the letter and brochure. They may be the outraged ones.
Having been shunned by my father, it was never outrage that I felt. Why do JWs think that is how their shunned kids will feel. Rather, it is an overwhelming sadness but also with an understatement of why this is happening, something that many don't understand.
It is the Watchtower along with the Governing Body that is responsible for this. Any outrage is directed towards them. It is those men that demand exclusive devotion to them over the natural affection that JWs have for their own families. They break up families.
I am sorry this has happened to you. I am also sorry this has happened to your parents. It will be just as hard for them (and in some cases, maybe more because it's not natural). There will be lots of tears.
It saddens me to see that they couldn't even sign the letter with 'love'. It's a twisted, evil organization.