Although people shouldn't count on bonuses, a lot do. I wouldn't reduce it to give her something she may not want. Maybe they get free passes or discount or something and she'd really miss the $50 more.
Is it in your budget to keep the bonus the same, and give the zoo membership as a gift to her kids? DS is 2, and we use our membership all the time. I'm sure they would love that, I think it would be a great gift.
Yeah, definitely do not decrease the cash bonus. IF you can swing an addition of the zoo membership that is nice, but not a substitute for cash in this circumstance.