1. Choose the time of day for errands very carefully.
(Not in the middle of naptime, lunchtime or a nursing session.) My favorite is right after breakfast, but that means, you have to have prepared your lists, diaper bag, stroller and such the previous night.
2. Involve, involve, involve the children!
(After Mommy started doing this, my oldest commented, "This is so fun. And, being here went SO quick this time!") It will seem like it takes longer to have them "help," but there are fewer meltdowns and meltdowns take time, so really, I think it evens out. By the way, involving the children makes it easier not to lose them. And, don't forget to involve them with the carrying in and putting away also.
3. Outline the Walmart/errand rules before you head inside.
We like "Don't whine or beg." "Stay with Mommy and the cart."
4. Never, ever, ever lose your head, your cool or your patience!
(Who will take charge if you lose it, your 4 yr old? I think not.)
5. Reward with something small for good Walmart behavior.
Our favorite is a pack of gummies after Mommy loads all the groceries in the truck.
6. In the event of disobedience/breaking the rules, quietly remind of the rules and remind that consequences will come later in the vehicle.
In extreme cases, park your cart at the service desk, go out to your vehicle and take care of the matter. Then, return to your shopping. It is inconvenient, but worthwhile. Sometimes the consequences simply have to wait til you're at home. Our society is no longer conducive to almost any form of public discipline. So, it is up to the you to make the best of an embarrassing situation, and later, at home, teach how to act in the grocery store so that you don't experience a repeat meltdown.
7. Beware of parking lots.
Put your kids in the vehicle first. Then, strap them in and then load your groceries. Groceries can be replaced. Kids can not. Be extremely watchful in parking lots.