I think you should NOT change your rate. it just shows you are flexible on your rates and you run a business based on inconsistant policies.
Just think come the middle of winter. -40 degrees and snow blowing all over. Who wants to go out in that? It doesnt matter if you are walking 5 minutes or 45 minutes, just bundelling up small children is a challanging task on its own and could take you 20 minutes alone. And I would think, geez, Im doing this for a measley $15!!
The parents are lucky to have found you right on their street. If it were anyone else, they would have to drive furthur to pick up their kid and still pay the same if not more.
Some providers have a set rate whether dck is using the before school option or not.
Get ready to expect them to ask you to care on PA days since you are so close to them too probably.
I dont do before and after, but I would charge $15 as well. Even if it just the afternoon portion.