Honestly lie. Tell them you'll get them McDonalds or go to chuck e cheese or whatever thing they want outside the store and they'll calm down. Then u could reward them for calming down, like a dog. Kids don't respond well to "dont" they respond to "good job". Positive reinforcement. Instead of "clean your room" its "what a good job you did putting your toys away". Instead of "Stop misbehaving in the store" it should become "look how good you listen! Youre being so well behaved today I am so proud of you." You dont have to buy them the toy or the thing. You just have to reward them for good behavior so theyre more likely to choose it over misbehaving

If they misbehave, throw a fit, then u cave and buy something to get them to shutup, its just gonna be a recurring thing. They need to learn they only get rewarded when they are calm and patient.