I say expose them. I think that when things aren't hidden from them, they understand it better. My daughter was first exposed to death at 4 when her Grandpa passed from cancer. We hid nothing from her about his illness. She knew grandpa was sick and going to die. When the time came she handled it better than the rest of us.
She has been right there twice, when we have had to put a cat down, holding them and petting them, because she wanted her to be the last thing they knew. There's no way I would have told her she couldn't be a part of saying goodbye, even with how hard and terrible it was.
So when she wanted snakes (at age 6), we explained to her what and how they eat. And she's totally fine with it. She understands her snakes need to eat. And she plays with all the rats from the time they are pinks, until they get fed off. She feeds the snakes sometimes as well. She gives the rats a head scratch, then kisses them, and then puts them in the tub.
Give them some credit, they will amaze you.