My opinion is that the Basic Game (3.5) boxed set is probably too much for kids that age.*
I used the (3.0) D&D Adventure Game boxed set to introduce my sons to D&D when they were 8 and 11, and they ate up every bit of it. I would highly recommend it if you can still find it, and I think it was only $10 or $15, worth every penny.
The next thing we did was a d20 conversion of Palace of the Silver Princess, and that was a blast, too.
*That being said, it is (was?) one of the best values in 3.5 d20. In fact, I bought both versions of it. Each came with a pre-gen adventure and characters, matching minis and monster minis, dice (IIRC), and several double-sided dungeon tiles. If they kept making these, I would have dropped $25 on them every time.