This is a great dog walker, keep her happy…pay up.
Is she asking to be paid?
This was my Contracts professor’s favorite hypothetical of implied contracts in my Contracts class:
Someone unexpectedly knocks on your door. When you open the door, the strange visitor starts playing the violin. When he’s done playing, he demands payment. Do you pay him?
You had no contract, so no.
If you had a reasonable opportunity to close the door or tell him to get lost, cutting off your receipt of the benefit of his violin playing, but you sat there and enjoyed it anyway, then maybe you have an implied contract to pay the reasonable value of the services, but you had no way of knowing he would demand payment at the end of the song.
The dog walker made a unilateral mistake without your knowledge or ability to take advantage of her mistake or prevent her performance.
If I was the dog walker and I had a schedule to follow and I blew it by accidentally performing an extra off-schedule walk, there is no way in hell I would bill the client for it. I’d eat the loss.
If the dog walker can’t understand this concept, then get a new dog walker unless, in the long run, her deal is too good to pass up and you want to keep her. Then you might pay her half or the full amount, but only this once, with the understanding that she will get no more payment for any future unscheduled walks. If she really is cheaper than other dog walkers, then in the long run, the money you save will more than pay for the one-time loss.
Only consider paying something if she expressly asks for it. Don’t volunteer payment for the unscheduled service.
I don’t care if she needed to rearrange the schedule to accommodate a better walk. 
First of all, she should ask me to do this first, and second of all, I get her to walk the dogs ONLY on days we cannot. Why would I pay for a walk on a day where I or my husband could have done it ourselves? She can’t just go around switching up schedules to suit HER needs. I’m a paying customer.
Regardless, I’m going to pay if she asks for it. She’s too good to make a fuss over this, but I will mention it the next time I see her.
I charged $15 for one hour walk. I’d go around with my van and pick up all my dogs, I’d usually have between 7 and 10 dogs. Then we’d drive to the area where we walked, I had a few different spots. They’d all pile out and run around for an hour, then back in and drop them off at home. Between the picking up and driving, it was longer than one hour in total. I had a morning group and an afternoon group so I’d do the same thing, with different dogs, twice a day.
It was the best job I ever had. I spent the whole day laughing.
The correct answer is to say 'Thank you for the lovely gift of music!" just as they finish,then slam the door. 
When I was covering as a dog walker, I always viewed me showing up on a day (or for the 4th time in a day when it was supposed to be 3) as my mistake and never expected payment. I was also on an exercise kick, so it really didn’t matter to me at the time. However, if you ask me to come 3x, and you only remembered 2x, you better pay for that third time.
Did she ask you to pay her for it?
I would be inclined not to pay her because of the scare of having my dog disappear and not knowing what happened to it right away. Sure it worked out the first time she got her schedule mixed up but if you continue to pay her for messing up the schedule and she continues to do it what will you do then? If you feel she deserves something then I might offer her a gratuity for it but I would not pay the full price for her mistake.
Holy crap! There are people that pay someone to walk their dogs 4 times in one day? WTF? Those sound like people who don’t have time for animals and shouldn’t have them.