My mum is a PITA for promising to give gifts then not following through. Still waiting on a washing machine for my wedding present - I got married 12 years ago, have had 2 kids and a divorce since! She also promised to give me a diamond necklace for my 30th - I got a £25 tea towel!
Disclaimer: I didn't ask for a washing machine or necklace, I never ask for anything, but when someone promises one it would be nice if they kept their promise. And I smiled sweetly when I got the tea towel and said it was lovely (it was fucking ugly)
She now lives abroad and because of the cost of postage and risk of breakages (I usually buy her perfume you can't get where she lives) we just buy for each other when we see each other - but she would usually buy gifts online to be delivered to me for the DC in time for Xmas.
2 years ago she decided I was too old for presents and stopped buying for me, so I reciprocated. She still bought for the DC though and this year even picked out some Lego sets for them which I said would be perfect.
I'm a teacher and a single mum so any parcels I order I get delivered I send to the school as no one is ever home to take them in and my neighbours all work FT too. So I told my mum to send the Lego sets to school. Our last day is Tuesday and the building will be shut over the holidays so I messaged her yesterday to say if she hadn't sent them already to send them to my house no the school.
Anyway, she's not getting the kids presents this year. She's re-negged on the Lego. I asked why and she said she will just buy for them whenever she sees them next.
I know my mum, we've had a fraught relationship and I strongly suspect it's more to do with the fact that she doesn't think I've called her enough this year (she's implied this, she never calls me despite being retired, I always have to be the one to call her she never calls me). She punishes me in little ways like that.
To clarify: her and SD have no money issues, they sold a succession of homes for huge profits and live a very luxurious lifestyle, wear only designer clothes etc. £30 Lego sets would not be an issue.
The thing that hurts most is that she is buying for two of my siblings kids - I asked her, DB is a 'struggling single father' you know 
(a single father who doesn't pay maintenance and actively reduced how many times he saw his DC) and DSis has been 'super supportive' this year
my other DB isn't getting one for his DD though. "I'll buy for her when I see her". And she also had bought for her best friend's adult children (she told me 2 weeks ago she'd got them Debenhams vouchers and was panicking that they'd be useless).
AIBU to be really fucking upset by this?. She often uses gifts as a punishment or bribe. For example, when I got married she said she'd buy my dress. I found a total bargain at £300. She said she'd only buy it if I started sending my SD's family birthday cards (I've never met half of them, the other half i last saw when I was 8 but she likes to play the dutiful wife to his misogynistic family). I said no BTW and bought it myself!
I feel like if she has an issue, speak up, don't take it out on my children.