1. I LOVE your little details with the mouse ears, the shell, and the sword fish. The crew did a fantastic job getting those to stand out, your edge tile, too! It makes your pool so fun.
2. Be thankful those were 'little' friends, hopefully under 50lbs, so they didn't damage the plaster with their weight. You'll have to check that area in the spa you said was drying. Hopefully it wasn't splashed.
3. Brushing will help with the mud. Are your pumps on? Check with your pool builder - But that's what I'd do. Let the pumps and water flow take that dirt to the filter. It may look bad, but that's because you don't have chemicals in the pool. I doubt it'll hurt your plaster. Like you said, your plaster's dry, you could stand on it. I doubt dirt will dirty it -- Not when you have plaster dust hidden in the cracks. So my thought is, don't worry! Dirt will wash down the drain!
Gosh, maybe you should post caution / closed signs on your pool. Why do kids do this now? Did they sneak and mess around before the plaster? Curious.
4. Keep the water flowing. We noticed about 3" gain in an hour with 3 hoses going and stayed up all night. You don't want a water ring!