Mine always liked to follow the school tour children. These animals are cute as the dickens, but they don't stay small very long Little calves will get scours if you over feed them, and that is what will likely happen at a petting zoo. So you have to watch out for this. The calf pen will have to be cleaned daily and should be of an extra generous size with a grazing area. You will want to get replacement calves and lambs each year, so you will always have small animals.
Children love small baby lambs, especially if they are born at the petting zoo.
Have several of them if you can. We all know that animals have to have a lot of fresh water and adequate feed. Families want to see animals in a nice pasture or at least in a corral that is neat and clean. A well thought out petting zoo will help you avoid problems associated with animal care. But even with the best of care, animals get sick and sometimes injured in some way.