Years ago when rearing my children, I used a technique that saved money and my own sanity from shopping tantrums.
When a child wanted something that I was not prepared to buy, I would cheerfully agree that it was indeed, "wonderful", "neat", or "cool" and that we should remember to get it "when our ship comes in". I never said "No", which is disappointing, and has such a final ring to the subject. Sometimes they asked when the ship was coming in and I'd say, "I don't know, but I sure wish it would hurry up."
The key phrase was "...when our ship comes in", (which was an old time phrase for ,"when we have money").
Using this technique validated his/her ability to choose, and gave hope for the future while teaching the ability to withhold need for immediate gratification.
Thirty years later, my son said he'd always imagined a tall ship coming toward him and wondered when it was ever going to arrive. This worked like magic for me. Not only did it create family harmony but it saved an awful lot of money.
Source: I read this in a letter to the editor in a popular women's magazine in the 1970's. I cannot remember whether it was Woman's Day or Family Circle.
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Whenever I take the kids shopping with me, I always park near a cart corral. This way, after shopping, I do not have to walk all over hunting for a place to put the cart, plus the kids can see me from the car.
When shopping with young children please use the seat belts and child seats in buggies. I have seen children almost fall out or climb out themselves.
When grocery shopping with my younger children, I would give each of them 2 quarters to spend in the gumball machines at the end of our trip. While shopping, they would see all kinds of things that they didn't need, like a certain cereal just because of the toy inside or some over priced item in the toy row...