The Witch likely thought she could subvert them all, and thus beat the prophecy. I mean, Edmund sells out for a box of pastries. She likely thought the other three could probably be just as easily bought off. Here's your heart's desire, now do my bidding (fits into the Witch = Satan part of the allegory).
Another angle is she really wants Aslan. I'll spoiler this part, since it's partially from The Magician's Nephew
The Witch has been in Narnia since its creation (she's literally there watching when Aslan makes it). This is why she knows Aslan and the power he holds. With the children in her grasp, she can get rid of her true nemesis and be done with the person who made the prophecy in the first place.