This question was prompted by a prior question where the OP happened to mention that they found zoos depressing but didn't want to tell their kids. I felt the same way as well, yet like other parents, I took my kids to the zoo.
My concerns centered around how bad zoo conditions were for those living there, and the ethical questions surrounding keeping animals in captivity for amusement (studies show that families use zoos to teach their children about social mores, that is, values centered around human needs - like the importance of family-centered activities and social bonding - not for the education of kids about animals.) There was disrespectful behavior towards animals by other zoo-goers. Often the hygiene was terrible. Animals were on display who had obvious signs of abuse (bite marks, lacerations, etc.) from aggressive cage-or-aquarium-mates.
There are some who postulate that taking kids to zoos decreases a child's empathy towards animals (whereas pets have a positive effect on childhood empathy), and the few studies claiming that zoos have a positive effect on attitudes towards animals are largely financed by zoos themselves.
This isn't meant to sound like an anti-zoo rant (I'm sorry that it does.) As I mentioned already, we took our kids to zoos a lot until perhaps 9 years of age, and they are certainly an ingrained and treasured part of childhood in many cultures.
I just wondered if there are any good ways parents know of to teach our children empathy for animals and lasting concern for their welfare - the purported reason for the existence of zoos - instead of just being quietly depressed?
There were obvious signs of stress and boredom in the animals, but I didn't exactly point that out to my kids. The education my kids got at the zoos was only superficially about animals; I didn't think it right to educate the kids on abuses of animals in captivity, the trichotilomania and head banging in apes, the treatment of animals with Prozac and behavioral therapy, or the decreased lifespan of animals in captivity.