"We are not buying that today."
This is close to our response.
Kid says, excitedly, "Mommy, look at XX! I want XX! Let's get XX!"
I say, "Oh, you really like XX, huh? Thanks for telling me. We're here to get YY today, though, so let's find YY."
Just the other day, our youngest (age 4) put shoes on in a store when we were buying shoes for his sister. Youngest already has shoes. I let him try them on and told him that yes, they were cool Batman shoes. I reminded him that he has cool Ninja Turtle shoes and that we were there to get shoes for Sister and socks for Big Brother. Then I walked with him down the shoe aisle as he wore the Batman shoes. Then I helped him take off the Batman shoes and put them away. I did let him pick out some new socks, though, because I honestly don't know what happens to all his socks.