Rmd to LaTeX without preamble

child.Rmd

---
title: title
author: author
output: html_document
---

# Section header

Body text goes here.

```{r pressure, echo=FALSE, dev=c('pdf', 'png')}
plot(pressure)
```

child.tex

\section{Section header}\label{section-header}

Body text goes here.

\begin{figure}[htbp]
\centering
\includegraphics{figure/pressure-1.pdf}
\caption{plot of chunk pressure}
\end{figure}

run_this.R

knitr::knit("child.Rmd")
rmarkdown::pandoc_convert("child.md", to = "latex", output = "child.tex")