Gift giving is an art. You need to give thoughtful and appropriate gifts without being either stingy OR overly generous.
Because being over generous places an unwelcome obligation and embarrassment on the recipient, it’s just as bad mannered as being stingy. It’s “playing lord/lady magnanimous” and can be more about the giver stroking their ego than providing genuine pleasure to the receiver.
I think it was in particular poor taste to give the niece such a generous gift, that can very easily look like (even if not intended to be) an attempt to buy the child’s affections which is never an appropriate way to behave.
You’d only been with him just over a year, you barely knew these people and you certainly weren’t close enough to them to act so presumptuously. I also think it was rude to exclude the sister but acknowledge the niece, that could well have been interpreted as stepping on his sisters toes as a mother. I can just imagine the aibu written by the sister!
“Aibu to think brothers new gf overstepped? They’ve only been together about a year I barely know the woman and dd certainly doesn’t, she got me absolutely nothing but got dd a rocking horse! The gifts we got dd barely got a look in! I didn’t say anything to her or of course in front of dd, but I was really hurt and it really put a dampener on my Christmas”
But he’s at fault too if he didn’t say a word to you if he knew what you were planning to buy/give because
he knows them far better than you and likely knew what their response would be.
I disagree with other posters that this early on joint gifts from you both would have been appropriate, that would have looked like you were trying to appear too “coupley” too soon.
But 2 years in yes it should be a joint effort with him guiding on type and level of expenditure.
However, I also agree if this early on he’s expecting you to take on the “wifework” of choosing, physically getting and wrapping gifts that’s a bad sign too, and also being somewhat manipulative regarding gifts isn’t good either.
The recipients wouldn’t have necessarily known how much you spent either and so the gifts could well have been believed to cost more than they did.
Even £40 I think is a fair amount for people you didn’t know that well. How much is that machine in normal shops not on offer? Because that’s what they might have thought you spent. A quick google suggests they normally retail for between £60-80. In my experience that’s a lot of money! If someone had got me something like that and if only got them something more the. £10-20 mark I’d feel shit!
As for the rocking horse, you never get items that are large physically for children as you don’t know if the family has the space for it, you don’t buy noisy or messy toys unless you’re intending to piss off the parents (for a while my sister and I had a good natured “wind up” going where we’d get the dns messy/noisy toys) you don’t risk outdoing parents/Santa either, and you certainly don’t bump the parents and spend more on the dc - for future reference.
You are quite young (I know you don’t think you are but honestly you are) so you’re still learning this stuff and it is a learning curve, but do learn from it, not just for this boyfriend but any future different ones too.
Because even if they and their family are better mannered and more understanding than this current one and don’t comment, they might well if you made the same faux pas again and that would affect your relationship with them.
You want to get people gifts they like and would genuinely use/welcome, that is in roughly the same price bracket as they’re spending on you, that is appropriate for you to buy based on how close the relationship is.
There are tons of online guides and articles on this stuff you may find it useful to read a few.