Although I understand that you acted from a good place, the children were never in danger, and you saved the girls from some anxiety, I would be pretty upset if this happened to my child.
If the bus driver let the girls off with you, then he likely would have let them off with other people, some of whom might not have been "right". I wouldn't ever want my child let off the bus to someone who I had never met, and I'd worry that by taking these kids you'd communicate to the bus driver that he could let your kids off with strangers in the future.
Let's imagine that the kids had gotten on the wrong bus, instead of off at the wrong stop -- how would mom have known where to look? How would you have known where to find them? Put them in your car and driven them back to school? Taken them to your house and called the police? It could easily have become a nightmare.
Why not ask if they know their mom's cell phone number, and let them call. If she says "Oh, I'm on my way" then ask her if she wants you to keep them. If she says yes, then all is good. If they're too young to know her number then no way would I want the responsibility of keeping them, because how would you return them?
If you'd seen them even once, and had a good sense that they were in the right neighborhood, or if they could have told you how to walk to their house, or if someone else (the bus driver, the other mom there) could confirm that they were definitely at the right stop, then I'd feel differently, but in your case, I would have either put them on the bus, or had them call right then and there.
Having said that, as the mom, my anger would be directed at the driver and the system, not at you.