How do I knit child documents with parameters into a main RMarkdown document?

I have a parameterized RMarkdown file, parameterized.Rmd, with the following contents.

---
title: "Parameterized report"
output: html_document
params:
  input_df: NULL
---
```{r sec1}
head(params$input_df[, 1:2])
```

I can knit it from the console using rmarkdown::render, generating distinct documents for distinct dataframe inputs. This works as expected.

rmarkdown::render("parameterized.Rmd", 
                params = list(input_df = mtcars), 
                output_file = "cars.html")

rmarkdown::render("parameterized.Rmd", 
                  params = list(input_df = iris), 
                  output_file = "iris.html")

I would like to have each of these results be child documents to a common document. My first attempt is with knitr::knit_child, but it does not take params as an argument. So this failed.

---
title: "Main report"
output: html_document
---


```{r test-cars}
knitr::knit_child("parameterized.Rmd", envir = environment(), quiet = T,
                  params = list(input_df = mtcars))
```

How may I knit together child documents which require parameters?

Solution

What worked for me (derived from the documentation once I properly understood it.):

Instead of using the params field in the YAML header, set the values of the parameters inside the main document and call cat on the output of knitr::knit_child. The below files achieved the desired result.

parameterized.Rmd

---
title: "Parameterized report"
output: html_document
---

```{r}
head(df[, 1:2])
```

main.Rmd

---
title: "Main report"
output: html_document
---

# mtcars
```{r mtcars, echo=FALSE, results='asis'}
df <- mtcars 
cat(
  knitr::knit_child('parameterized.Rmd', envir = environment(), quiet = TRUE)
  )
```

# iris
```{r iris, echo=FALSE, results='asis'}
df <- iris 
cat(
  knitr::knit_child('parameterized.Rmd', envir = environment(), quiet = TRUE)
  )
```

Knitting main.Rmd applied the parameterized report to each dataframe.