I put together this simple Ms-Powerpoint presentation to explain Roman Numbers to year/grade 3/4 kids. My daughter loves it and is now empowered to compute any Roman number to its decimal value or vice versa.
There is no need to memorise charts of Roman numbers. Simply make kids understand the logic (or bottomline - as I tell it to my daughter!) and it will all be easy. I think this is quite enough for the level that grade 3/4 kids need to know. They can do more thorough research on Roman numerals, as they grow up of course!