Depends on when and where they hit it. If they wait until the satellite is low enough that it's beginning to contact the atmosphere, then most of the little pieces resulting from a missile strike will burn up pretty quickly.
I would think -- and I admit I'm no physicist -- that it would depend in what direction it's hit. If it's hit directly from below, that might blow the debris back upward. But if it's hit obliquely from behind -- ideally behind and above, it they can target the missile that precisely -- then it would drive the debris downward, where it will burn up in the atmosphere.