Good starting ruleset for a 9 yr-old?

[Particle_Man]

I would also suggest C&C, since it would be a "bridge" between Basic (which he has, and which you have monsters for that can be easily converted (subtract AC from 19, decide if monster is good at physical saves, mental saves, both, or neither, and you are there), and 3e (if you decide to later convert, it is easy to say "it is like C&C, but has extra things called feats, and combat has more maneuvers, and you can spend skill points, etc.")

At least, imho.

 

[kengar]

As far as basic D&D goes I liked Mentzer a lot more anyway.

Mentzer was geared more towards folks who were completely new to gaming, and possibly a younger audience as well. It included the solo dungeon with sample character, etc. Moldvay was organized more clearly as a rule book, IMO.

I did like the DM info being in a separate volume w/Mentzer, but that was offset somewhat by having everything in one book with Moldvay.