Abby found a rock.Good voice, nice pace.
It was the size of a plum, dark gray, and roundish.
I would like to pause and just remind writers that editors really can tell stuff like this from your first two lines. Already this manuscript has made a good impression... and there are plenty of other manuscripts that I've rejected based on the first two lines. Never underestimate how closely your editor will be reading what you've written.
A very nice rock. Abby took it inside and washed it with soap and water.I don't think you need that last sentence. Most kinds will feel a rock that doesn't have obvious dirt on it is "clean." And this story isn't about relative cleanliness, is it?
She put the rock on the table next to her plate.Cut that last sentence, too. You're handing it to us. Show, don't tell. It's ok if she just puts in on the floor without comment and it takes us the next couple scenes to see that the rock is a pet/friend.
“Abby, rocks do not belong on the table.”
“You’re right, Mom. Pets should eat on the floor.”
“That is not a pet,” Mom said. “That is a rock. And it’s dirty. Please wash your hands.”You could cut these lines, too, I think.
“He’s clean, Mom. I gave him a bath.”
The next day during recess, Abby sent the rock down the slide. She buried it in the sandbox. She pushed it on the swing.Nice.
Walking home from school, Abby heard a sound. Rumble, rumble. But when she looked, nothing was there.The transition here from street to indoors was confusing. Does she need to hear the "rumble rumble"? Other solution?
“Abby, come here!”. Mom was pointing into Abby's room.
Abby's eyes opened wide. The floor was completely covered with rocks. Big rocks, small rocks, tiny rocks. Bumpy rocks and smooth rocks. Rocks of every shape and color.
Abby saw the open window. She remembered the rumbling. “They must have followed me home from school.”
"Don’t be silly,” Mom said. “Please pick them up.”
Abby piled all the rocks in the corner of her room. “Maybe I’ll start a rock collection.”
Well, I'm certainly turning the page. Good storytelling, overall. I'm very curious about how this wraps up. If it's clever, you may have something quite publishable here.