AIBU to refuse gifts from my parents / in laws.

[NattlesNat]

I have a 2 year old son, he is the first grandchild on both sides, and so my parents and the in laws want to spoil him. This is lovely, but it’s the type of gifts they get that drive me mad. They are all about spending, spending, spending with ZERO thought. Things they have bought;

1. cheap tat from abroad with loose parts, choking hazards, button batteries, etc, that’s all completely age inappropriate.
2. duplicates of stuff I’m guessing they don’t know we already have (they never bother to ask).
3. duplicates of things they know we already have (e.g. two play hoovers, two doll prams, two bubble machines.) they explicitly say they got it because they know we already have one but “this is a slightly different version / style”.
4. stuff we would never want (e.g. a ten foot trampoline when our garden is only 20 feet long and wide).
5. hampers of chocolate (our son in allergic to dairy, which they know).

I’ve tried politely reminding them no gifts are ever expected, that if they really want to buy him something to ask and we can at least tell them if we already have it / whether it’s suitable. When niceties didn’t work, I bluntly told them “he already has this, you can either take it back or I will sell it (second hand for less than you paid)” to which they told me to sell them. But yet it still continues. I am honestly sick of having to do a big display of gratitude (they expect it) for these endless useless gifts. Can I just refuse to accept things now? How can I make them get the message?

This.
In the bin.
In front of them.

In future you could intercept each gift before your child sees it or before they give it to him, then hand it back to them as they leave.

Every time

Buying chocolate hampers when they know kids can't have dairy is unkind. That would have made me see red as its thoughtless and mean

It's performative gifting, especially expecting a song and dance of gratitude for dangerous/useless tat, things your child doesn't need, or that you don't have room for. I bet some of it is they're buying duplicates that are, in their view, 'better' than what the child has already e.g. you've bought them a bike from the local independent bike shop but granny turns up with one which is the same brand as someone she's trying to outdo in the 'world's best granny' stakes, and therefore it must be better!
Not about your child at all. Everything to do with bragging to the other grannies at the bingo or women's institute.
If both grannies are at it, you've got a turf war going on! Again, nothing to do with the child, everything to do with trying to be the best / favourite granny etc etc.
I would be handing gifts back with the line 'oh lovely, you can play with this at granny's house! Then you'll have one here and there!' Clearly not with anything that's cheap dangerous tat. Anything that the child is allergic to gets given straight back with 'oh dear, we can't have that in the house because of x allergy. Do enjoy it yourself!'