This doesn't seem like a package installation problem, since you don't mention a problem with either install.packages() or library().
Also, as far as I can tell, there is no dataset called children in the package ca, so this problem most likely has nothing to do with the ca package at all.
Instead, the error indicates that the file children.csv does not exist.
If the file does exist on your machine, try to specify the full path to this file when you do the read.table(), e.g:
read.table("path/to/children.csv")
Finally, note that you can use read.csv() to more easily read CSV files.