If we are going to speculate that SW hit her head on a rock, we might as well speculate that she hit her head on any number of things that could have hurt her.
H is the one who said SW went under the water. He said he picked her up and asked if she was okay, and she laughed and said she was (per H). I think if she had blood running down her face surely he would have questioned her. Or alerted her mother and grandmother.
I think if SW had an obvious injury, CB would have gone to seek medical care of some kind. She could have taken SW to an urgent care or back to the place where CH went.
If SW had an obvious injury that needed tending (and in a place or type where "she fell" is a believable explanation), i think they all would have known to take her to the doctor. Especially if it happened in Kingsport in a public place.
Now, after riding her around for hours running errands and then going all the way back home if something harder to notice that might entail a more thorough examination and for which there is not an easy explanation (this would go to 'the she was hurt but they didn't know' theory) and which seemed to suddenly get worse.... That might be different. Because it would probably cause a look at her homelife and get them reported to DCS again.