Supermarket, toy section.
Kids, «I really want the fire truck lego set!».
Don’t say, «I already told you we’re not buying it, you have already a lot of Lego».
Say, «I know, right? It’s really cool! It even had a ladder that extended! Let’s go home and see if we can build something similar with our Lego!».
Repeat as many times as needed. You value their feelings, make them feel listened and avoid a crisis. ♀️