I've never sat any of my dc into the trolley. It's dangerous. It'll tip and they'll fall and be injured. It's dangerous.
I've had small age gaps and bigger ones and never put my dc in the trolley part. I've walked to the shops with a baby in a sling and under 2yr old, gone on a bus and walked with dc, used a single buggy, a double buggy, a car... still never seen the need to put dc in the actual trolley part. I don't get it- why would you do that? Absolutely no need
Unless you have someone to push another trolley to put shopping in?
My youngest dc would eat the food in there if I sat them in the main bit- and I need the space, to put the shopping? how can you get food shopping with a child in the space the shopping goes, sitting on eggs, crushing bread, messing with stuff...
I take a buggy- and carry a basket- for bits. Or used to fold the buggy up and put that in the trolley- dc in the seat. OR- two baskets, one on buggy, one carried- and weekly shop done like that. Really, really wasn't difficult.
I'm a great believer in reins for small children- especially in shops- my oldest was a runner when he felt the need and reins were essential, for my sanity and his safety. And also, online shopping for big weekly shops. But I have done it on foot, on buses etc and never put the dc in the actual trolley part- or even considered it- didn't used to see it either until a few years ago. Considering most shopping trolleys have seats I could fit my 8yr old in, I don't see why- harder to lift into the trolley part than the seat. I shudder to think what my ds1 would have done if allowed to sit in the trolley (besides the seats- with seat belts- which have been around a long time, because he's 16 this year!). I also don't let them eat in shops though either...