I'm struggling to understand the 'sour grapes' thing too, and the suggestions that the OP is somehow 'jealous' of Brooklyn Beckham.
Expressing the quite reasonable view that the Beckhams' linking this photographic exhibition to the Grenfell fire is distasteful and opportunistic does not make the people expressing that view 'jealous'. It means that they find the Beckhams' actions rather studied and cynical. I don't think that anybody expressing that view on this thread wishes that they were Brooklyn Beckham, or envies his success (which he hasn't had, let's face it).
I'm not buying the argument that the Beckhams 'using their fame' will encourage more people to donate, either. I don't think that the terrible tragedy at Grenfell tower lacks publicity, and it rather smacks of hubris to think that the hoi polloi wouldn't react unless shown how to be charitable by the Beckham family. It rather insults us, doesn't it?
If it's not clear what 'sour grapes' means, it comes from one of Aesop's fables. It means belittling something that you would otherwise have wanted but now can't have. Here is an example:
Giles Deacon and Jonathan Saunders are both British fashion designers. Both trained for years at prestigious London fashion schools. Giles Deacon made Pippa Middleton's wedding dress. Jonathan Saunders has dressed a number of celebrities. Both of them are talented and creative. Both have closed their ready to wear businesses in the last couple of years.
Both of those designers might look at Victoria Beckham, untrained, with no fashion design skills (she couldn't design a pattern and cut out a dress, for example, let alone stitch one) and think, why is she running a successful fashion house and holding herself out as a designer when she plainly isn't doing any of the designing because she doesn't know how? They might look at awards she wins for her ready to wear line, or her sales, or publicity she garners, and think, well, I wouldn't want that anyway because it's become a debased currency if people with no design training can pretend to be designers.
That might be called 'sour grapes' (although frankly I'd understand why they felt that way).