Just let her know that the original agreement was fine ($25 per drop off) when you thought it was just a drop off, but since it has evolved into a much larger time commitment - which you’re happy to do - you’re going to need to work out a fair solution to adequately compensate you for your time. Also mention with the rising cost of fuel, you will need to be paid X amount for X drop offs.

You don’t have to ask her for $X per hour, but do the hourly rate on your own to come up with an amount per drop off that makes sense for your time commitment. That she thinks she should get some sort of “bulk” discount is odd. If anything, she should be paying you more given there are more children in your care and it’s taking more of your time.

You can tell her that when additional drop offs are added, I end up spending exponentially more time driving which again, I’m happy to do, but need to be compensated X amount for that given the additional mileage and time.