I'm sure this has been done many times.
I'm thinking about it today though 
If your child is too big to want or can't fit in the seat part you don't then let them climb in the other part of the trolley?
Filthy dirty feet where I'll put my food.
Standing up (dangerous)
I'm not being unreasonable to suggest that its the seat, or walk right?
I saw this with several families yesterday...it got up my nose 
Oh and I know there are germs everywhere... this makes it worse