catsize we must be from different towns 
When going to a child's party, you are always thanked at the handover, which is enough for me. For the majority of my friends and all family, at birthdays and Christmas, you've thanked at the handover or say it on the phone next time you speak.
I don't think the email was rude at all so YABU there but I wouldn't have needed that email either. So to expect a further note seems over the top and not even from the 4 year old, from the mum doing her duty, that's seems RU to me.
I am not against thank you notes if that's your thing, but I am against people thinking that's the only way to say thank you and that people who don't send them are ignorant. The whole 'proper' thing is for your own benefit, not the recipient as you have no knowledge of if they feel thanked or not. Unless the recipient thanks you for the thank you note and omg where does it end?
My friend is a serial thank you note writer on behalf of her kids. When I give her child the present, she doesn't say thanks, she opens it, then without fail, shakes the wrapping paper and looks in the empty paper to see if there's anything else and on one occasion even muttered 'is that it?' (£12 hello kitty sparkly top from next). Now, I got a thank you note from her mum, but it only made me think 'yeah whatever' and not the 'oh isn't that lovely and the done thing' that it seems to be built up by some posters. If the child had said 'thank you shakes' when I handed it over, it is case closed.
So OP, have you sent thank you PMs to everyone who posted on the thread? Haven't had one for my earlier one and am feeling PO 