Two utilities in package taRifx can be used in concert to produce multi-row tables of nested heirarchies.
library(datasets)
library(taRifx)
library(xtable)
test.by <- bytable(ChickWeight$weight, list( ChickWeight$Chick, ChickWeight$Diet) )
colnames(test.by) <- c('Diet','Chick','Mean Weight')
print(latex.table.by(test.by), include.rownames = FALSE, include.colnames = TRUE, sanitize.text.function = force)
# then add \usepackage{multirow} to the preamble of your LaTeX document
# for longtable support, add ,tabular.environment='longtable' to the print command (plus add in ,floating=FALSE), then \usepackage{longtable} to the LaTeX preamble