The issue isn't so much full body strength as it is applied force. A consistent amount of force would have to be applied for a sustained period of time to perform the strangulation. This might be beyond a frail 9-year-old's capabilities, but who knows. My guess is that with her body already shutting down from the blunt force trauma, the strangulation would not have taken as long to end her life.
I've seen this brought up a few times now and I'm confused. My understanding was that the whole point of using the paintbrush was less about it being a handle to be pulled, and more as something that could be twisted to tighten the cord and then would hold itself in place. The strength of the one applying it would then be pretty much a non-issue.