Not much I can add to the 2 previous posts. V&A would certainly be kid friendly if the opportunity arose. But in all the meals we've had there since 1988, I have only seen children twice--one girl (10-12) with her parents, and one girl (again,10-12) with her mother. In all honesty--they both looked quite bored. Our meals usually last at least 2 hours, and more often 2 1/2--a long time for a child to sit at the table.
But--you do know your niece and if you do decide to take her, you can count on your Victoria and your Albert to pay a lot of attention to her.
Regarding the menu--there is no children's menu, so she might be very limited in what she might like to order. But I suspect (tho I'm not sure) that they might be able to round up something else from Citrico's next door--provided they are open that nite. To find out for sure, you can call V&A direct at 407-824-1089.