My message to the mother:
"Sorry mom, it might be an inconvenience to strap the toddler into the car seat, then put her in a shopping cart, and then back into the carseat, but having the baby, you kinda volunteered to take care of them."
I have a 17 year old, a 13 year old and a 6 year old. I would NEVER leave my 6 year old with my 13 year old and rarely do I ever leave her with my 17 year old.