I've been gaming with my kids using the Savage Worlds system in the Necessary Evil superhero setting. In Necessary Evil, aliens invade and wipe out all of Earth's heroes, leaving only the villains to stand against them. The setting is fantastic, and Savage Worlds is an intuitive and easily adaptable system. My kids are now 9 and 12.
The Savage Worlds Deluxe Explorer's Edition corebook is only $10, and the Super Power Companion is only $20. The "Explorer's Edition" is unabridged, but paperback 6x9 instead of hardcover 8x10.
My next choice would be the D6 System. Printed books are wickedly hard to find, but the PDF files are a free download at Critical Press Media and several other mirrors. The system is intuitive, easy to learn, and easy to adapt.
Neither of these require the level of math or bookkeeping that is required of TSR's Marvel, Mayfair's DC Heroes or several other RPGs.
If you want the current "branded" superhero games, Margaret Weis has produced Marvel Heroic using a unique system, and Green Ronin has released DC Adventures using what is essentially d20/True 20 (and they preserve the exponential tables that made Mayfair's line so popular). I've not played the Marvel game, but True 20 is very crunchy and strategic.
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