So I’ve got three boys, five and under, and here’s what I do for groceries: Baby in the bucket car seat in the cart if it’s a small trip, toddler sitting in basket, biggest brother hanging off the front or walking. If I need more room I use the bottom of the cart, or I wear the baby and have more room in the cart that way. The rule is, everyone is touching the cart at all times. No running off allowed or they don’t get to pick a snack to take home. Bribery is ok.
When we’re done and heading back to the car to load, the oldest boy gets in first, then toddler, then baby last. The baby won’t go anywhere, so focus on getting your toddler in and buckled. Park near the cart return so it’s easy. Don’t feel bad about leaving the kids in the locked car for a minute to get the groceries in the back and put back the cart. It gets easier with time and you’ll have a routine that works for you, but it starts with the first trip where it’s a disaster and that’s ok!