My young children have recently started being invited to parties and it really annoys me when there is no thank-you for the gift! Two of my DC are twins, and I always make sure to get a nice gift from each of them. On one recent occasion I even had something personalised with the child's name on. I saw the mum about 10 days later, and even in conversation the gifts were not mentioned. It's so rude!! A quick text is not that difficult, is it? Obviously I don't buy gifts because I want to be thanked, but I'd be absolutely mortified if I realised I hadn't thanked somebody for something they'd taken the time to buy for me or my child.
Anyway, I know I'm not being unreasonable. I just wanted to say that if this is you, it's bloody rude!