The trip up Mill Mountain was made, in retrospect, mostly so our kids could ride the "train" at the zoo. Said train is a group of little cars pulled by a riding lawnmower retrofitted cosmetically as a locomotive.
The animals were more impressive than what you'd see in your own neighborhood, but it wasn't all that impressive, though the kids liked the "train" so much we went on it twice.
If you live near Richmond, Maymont's Children's Zoo and/or nature center is a better bet, and if you are traveling on 81, going to the safari zoo near Natural Bridge is a much better investment.
My family went to this small zoo in the fall of 2008 during a trip to Roanoke. It's definitely not a big city zoo, but it's a great size for families with small kids. We were able to walk around, see the animals, ride the train and play on the slides. My son had a great time and so did we. The zoo is on Mill Mountain, near the Roanoke Star, and the view from the area of the surrounding city and hills is breathtaking in the autumn.
I thought admission to the zoo was kind of expensive for the size of the zoo, but it was still a really fun outing for our family.
Along with its great view of the valley, the zoo exhibits fifty species of exotic and native animals, such as red pandas, a Siberian tiger, hawks, snow leopards, and reptiles, like the Burmese python. Educational programming and fun can be found in the zoo’s amphitheater. The ZooChoo train is fun for smaller children. There are picnic facilities available. If you visit in the summer, check the schedule of live programs in the amphitheater.