But it's not 'giving quietly' is it, to use money you'd set aside for family presents then announce it to them! And apparently expected them not to mind. How would your son have felt if you'd donated the money for his presents too? Would he have congratulated you on this 'noble' gesture? Or might he have felt unimportant and side-lined?
Why didn't you donate SOME of the money to charity and buy (smaller) gifts with the rest. That way you need not have made a song and dance out of it. You need not have offended anyone or told anyone you'd donated it to charity.
You pleased yourself, and implied your sister/nieces/nephews are not worthy of the time and effort it takes to choose gifts.
You felt your charity of choice and desire to donate to it was more important than family tradition. This is the message they're getting from you.
Instead of donating money intended for other people, you could have asked for donations as presents, or asked for money instead of presents, and donated that. Or sold all the presents you and son received and donated the money to charity. That would have been noble. Giving up other people's presents isn't noble or generous at all!
Giving to charity isn't entirely altruistic. You get a rush of pleasure from giving, from knowing you are helping those in need. You had that rush of pleasure and warmth from your donation, at other people's expense.
So your family recieve nothing from you this Christmas, while you had the pleasure of donating their gift money to charity.