The school I work at often gets little chicks from the local chicken place (probably factory farm) and the class have to take turns looking after them or something like that. After a certain period they come and pick them up (ready for a life of misery and death).
Not sure if that is an option you could take. All the while you can be subtly dropping hints to them haha.
Its a sad world we live in that teachers like yourself aren't allowed to teach compassion and caring to young kids for animals. The only hope for Veganism in the future is for us to reach the next generation of kids before they have the status quo of animal abuse drilled into their heads.
I have to say I wouldn't go down the path of having animals in the classroom within that context. I work in a library and sometimes we have petting zoos in for childrens activities. I find it so upsetting that these innocent and trusting animals are treated so well for a few months, taught to trust humans only to (most likely) be sold off for slaughter once they grow out of their cuteness. It's also upsetting that the innocent children who play with the lambs etc. probably think that they go back to some lovely green pasture and spend the rest of their life there. It just teaches lies. Or it teaches kids that animals are disposable. Or that it is ok for adults to conveniently obscure what we do to animals. We lie to ourselves and then we lie to our children. If you don't want to perpetuate this, how do you choose an appropriate way of educating people?
As a librarian, I am all about freedom of information and what disgusts me is the misinformation that is propagated in many children's non-fiction items. We have books on our shelves that have titles like "Where does my dinner come from?" and they contain glossy pics of fat, healthy farm animals on picture perfect farms. Slaughter and the truth about factory farming is blatantly ignored, these books are pure fabrication.They go from cute lamb straight to chops with nothing in between. You don't need to traumatise them with Earthlings, but they should know the truth. One excellent book (possibly the only one of it's kind!) is "Meat: from the farm to your table" by Heather Hasan, it is a great non-fiction children's book that is both accurate and educational.
Maybe a way to introduce animals into the classroom in a positive way would be to get a friend who owns a rescue to bring their animal in and talk about its life, how they adopted him, about what it is to be responsible for an animal.
Ruby Roth has good resources too:
http://www.wedonteatanimals.com/home.html 
