Magic users, in my opinion, shouldn't be as good at fighter things as fighters. Not at first level, at any rate.
Here is the bonus for Toppling Spell:
Benefit: The impact of your force spell is strong enough to knock the target prone. If the target takes damage, fails its saving throw, or is moved by your force spell, make a trip check against the target, using your caster level plus your casting ability score bonus (Wisdom for clerics, Intelligence for wizards, and so on). This does not provoke an attack of opportunity. If the check fails, the target cannot attempt to trip you or the force effect in response.
I read it as gaining a trip if a target takes damage, so it would only apply once per hit. Magic missile's schtick is that it hits multiple times, so you would get multiple tries. If I were in a bad mood, I would say once per target, but even then you could point out specific targets for each missile.
Edit: That's why I never post here... I get ninja'ed while I'm trying to make my point of view make sense. :D
