G.R.
I would recommend Brookfield Zoo! I have seen other parties there, so I don't have first-hand experience, but I know they do family events in some of the restaurants near The Living Coast. I'm pretty sure you could even do a tented event or cocktails+ in the Savanna Terrace. I'm sure their event planner would have some nice ideas for you! And, its a great, fun venue in the summer--even early summer when its not so crowded and just coming alive for the summer crowds.
What a nice idea to throw her a party. Good luck with your planning,
G.