"The girl writes to the boy."
Translation:La ragazza scrive al ragazzo.
There is a nice table at http://italian.about.com/library/fare/blfare153a.htm
It only misses the compounds of "con" (only "col" is actually used, but optional).
Bambina/bambino mean young girl/boy, probably under 10. Ragazza/Ragazzo are generally used for teenagers, often up to mid twenties. Ragazzina/Ragazzino are also used for the roughly 10-15 'tweens'.
I guess it all comes down to context. It would be fairly unusual for an 8 year old girl to write to an 8 year old boy, writing letters is generally something teenagers do.
- Bambino is a young boy, up to the early teens
- Ragazzino is a boy in his early teens (if he complains about being called bambino)
- Ragazzo is a teenager boy or a young man (or even a man, uomo sometimes is a charged word)
Ragazzi is the plural of ragazzo, or a group with mixed genders.