There is a risk with everything we do. I assess that risk.
DC is sat down in the trolley. Just sat there. Its a bit like chucking in a whole weekly shop filling the trolley only my DC is lighter.
Its when kids stand, jump up and down or swing themselves about in it to make them move that accidents happen because they are shifting the centre of gravity and making them unstable.
If DC rocked back and forth or behaved in an unsafe manner - throwing their arms out, leaning over the side etc - I would not do it. But because DC sits in there calmly I did. A child sat in there is no more likely to topple the trolley than one heavily laden with shopping.
If my child just sat there will cause a trolley to topple then it will do so with shopping in it meaning they are unsafe for anyone to use and its would be even more risky to put two kids in the seat bit because they are that much further off the floor.
as for walking my DC round? hahaha! seriously. DC would fight not to go into a shop. Would lay on the ground like a dead weight and I couldn't shift them. Would kick and scream not to go in so yeah... I could've tried to walk DC in but it wasn't going to happen and we would've ended up living on junk from the corner shop because that's the only place we could WALK into without a meltdown.
nearly 10 years down the line we still have issues. We can manage around 15 minutes in a shop with precautions - ear defenders etc - before it gets too much. Yes, that's a flippin miracle for us. There are some shops that we still cannot go in even with an adult to hold each hand and precautions in place to lower over stimulation.