Zoológico de Quito is not in the historic centre or modern Quito at all; it sits out in Guayllabamba, where the warmer, drier valley landscape already feels like a different outing from the high capital. Its strongest reason for travellers is the focus on Ecuadorian and Andean wildlife, including species that are hard to understand from museum labels alone.
Treat it as a half-day side trip, especially with children, rather than something to squeeze between Old Town churches. The distance matters, so it makes most sense if you have transport arranged or are already exploring the northern valleys.