Hi Amy,
Thank you so much for visiting the Disney Parks Moms Panel with your question.
Victoria Falls Lounge is a lovely place to relax and take in the gorgeous atmosphere of
Disney's Animal Kingdom Lodge. I can understand why you want to take your niece there!
Yes, you can absolutely visit the Victoria Falls Lounge when you are at Walt Disney World. Because it is in a resort hotel and not in a theme park or water park, you do not need a ticket for entry. If you are driving, simply tell the security cast member on your way in that you are there to go to Victoria Falls Lounge. I suggest that you consider this visit for the end of a day when you are at Walt Disney World. Victoria Falls Lounge is typically open in late afternoon and evening hours, 4:00 p.m. to midnight. This might change, so do check the website I linked to check for hours on the day you plan to visit; however, I would plan this for an "after-park" activity. It will be a great place to unwind and talk about your day!
I hope that these thoughts are helpful for you, Amy. Please do not hesitate to ask if you have any further questions!