des petits toasts

Tom, in both places I see ‘des’ it means “some small (toasts)”, and fits. If it still doesn’t seem right to you, can you expand on why/where you think ‘de’ might fit?

I thought before an adjective 'des' became 'de'

Ah, of course! That didn’t register as I have heard “des petits toasts” used in France. 

I think in this case “petits toasts’ may have been treated as a “compound noun”,  Whether that is grammatically correct or not, it is in usage (at least as determined by asking my resident expert what she would say spontaneously) !

See links

https://www.tolearnfrench.com/exercises/exercise-french-2/exercise-french-4045.php

https://forum.wordreference.com/threads/fr-de-des-adjectif-nom-au-pluriel.136043/

On the word reference link, a fair way down is this :

Thank you Maarten, that's brilliant. I sometimes forget about these points of difference between written and spoken French. Very useful to be keep being reminded !