That would be:

La petite boutique des macarons.

Grammatically, la petite boutique de macarons is “more” correct but less stylish, in my opinion. It would also lead to la petite boutique de Macron these days…

Note that boutique des xxx is not incorrect. For example “The little shop of horrors” was translated in French by La petite boutique des horreurs.

Also: La boutique des saveurs, La boutique des vins, La boutique des voyages, La boutique des étoffes…

La petite boutique de macarons.

In my opinion (native French speaker), using “des” sounds incorrect/ childish