Discounts to the zoo are rare. If you are a member of either the zoo in Roanoke or Norfolk, then you may be entitled to a discount here. Our credit union sometimes sells tickets at a discount.
Your youngest will get in free. There may also be a student discount, but it wouldn't be much off. You may want to consider an annual membership--for your family it would be $59 which gets you in for a year. In addition, you get free or reduced entrance to a number of zoos and aquariums, like the NC aquariums. As a member, you are entitled to discounts in some of the gift shops, and maybe food venues (we're local so we don't eat at the zoo much).
Have a good trip--it is a really nice zoo.