It would be a great project for beginners, as a CMS is generally a fairly logical, straight forward system to assemble. Basic features present nothing terribly complicated and it would be a good exercise in learning database connectivity as well as basic text parsing. They'd either modularize it or learn quickly they should have
A modular system would be easy to add later more advanced stuff to.
I rolled a CMS, once upon a time -- the intent was to build something that was free of any database -- all flat file based storage for the purpose of portability. It worked well enough, but not being a consistent blogger I found the software got more attention than the actual use of it and abandoned it altogether