I'm multi-classing Paladin/Cleric. I can summon a poisonous frog with a level 1 cleric spell, and that frog can cast Smite Evil. Is my paladin able to activate the frog's smite evil as though I cast it myself (bonus to attack/dmg on first successful attack)?
No, but you should be able to rely on the frog activating it on its own. Summoned creatures may not be brilliant but they should do obvious things like use their powers.