Picture book about a treasure hunt for shells

I would have read this sometime in the 90s. I think it was about a family treasure hunt, where they were looking for periwinkles. It might have been someone's birthday and it might have been a yearly tradition for all the children to look for a shell. The youngest daughter really wanted to find the periwinkle but didn't have much hope. I think the cover was at least partly pink and the illustrations were a fairly realistic style/not cartoony.

Edited to add more details:
- although I think there are shells involved, I *don't* think it takes place on a beach, at least not entirely. I remember an illustration of a grassy hill
- Illustration was kind of painter-y
- Characters might have been dressed in Edwardian fashion

Maybe there was a key as well as a periwinkle? (Or possibly instead of a periwinkle because at this point I'm second-guessing everything!)

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Something by Alison Lester like Magic Beach?

Unfortunately no--upon thinking about it I remember the illustration style as being more painter-y kind of like Mirette on the High Wire maybe.

Additional detail I'm *really* unsure about: characters might have been dressed in kind of an Edwardian fashion (lots of lacy white dresses for the women and girls).

>5 keachachu: Do you have an idea of what country (if any) it might have been?

Not a clue! Given the possibly-imagined dresses, maybe England?

I should also have specified at the beginning: if there's a beach involved, the book definitely doesn't entirely take place on a beach--I remember an illustration involving a grassy hill or something similar. I'll update the post with these details!