According to their site, the child size is good for most dogs and cats, so that one would be sufficient, especially if you don't plan to work with many horses, neonatal (or very small breeds), or avian/exotics. However, they recommend the neonatal head in addition to accommodate newborn kittens and puppies. (also avian and exotic if your clinic sees those). The adult size is best for very large dogs and large animals.
Hope this helps,
Melanie