Wolfram Mathematica Demonstration Project has a free downloadable program kids can use to experiment with Roman numerals by adjustings sliding bars for ones, tens and hundreds.
Pick whether you will convert from Arabic to Roman or Roman to Arabic. Then select a level of difficulty. Numbers range from 1 – 3000 (I to MMM).
From site: This lesson illustrates addition with Roman Numerals. Notice how much more difficult it is to add when the number system does not use place values like the Arabic number system that we normally use.
From site: The Romans were active in trade and commerce, and from the time of learning to write they needed a way to indicate numbers. The system they developed lasted many centuries, and still sees some specialized use today.
LearningReviews note: This site explains the basics and has a handy Roman numeral converter that takes your number or Roman number and translates it.