A toddler can gross out a supermarket seat all by himself with his leaky poopy urine-soaked diapers, drool, runny nose drip, and sticky dirty fingers. Kitty is not any less sanitary than baby excrement/detritus.
Not only that, but I frequently see children sitting in the main part of the shopping cart. I guess their parents don’t have the faintest idea of hygiene. I don’t want their kids’ filthy shoes all over the surface that comes in contact with my groceries.