How to use child and plot properly following the reference exemple

I have tried to generalize the exemple given here :
in order to get a document whith a list of graphics for each column of mtcars data set.

So, I replaced the chunk in the child file

lm(mpg ~ ., data = mtcars[, c("mpg", x)])
plot(mtcars[ x ])
And knit the file, like the original example :

res <- lapply(names(mtcars),function (x) {
knitr::knit_child('DocumentCHILD.Rmd', envir = environment(), quiet = TRUE)
cat(unlist(res), sep = '\n')

It gives the following with no grapics at all ! The unnamed-chunk *.png files don't exist after "kniting" the master file.
Regression on "mpg"
img src="DocumentMASTER_files/figure-html/unnamed-chunk-5-1.png" width="672"
Regression on "cyl"
img src="DocumentMASTER_files/figure-html/unnamed-chunk-8-1.png" width="672"
... until on "carb".

What is the solution to have a nice succession of graphics by iteration on the list of mtcars column names?