Sorry I'm getting in on this late. I believe that I explained it once as a "last nap" or something like that where you go to sleep forever.
Birds & Guinea Pigs would be trickier, but we just kept replacing our daughter's goldfish with imposters until we thought she could handle the death talk. (I believe that was around 3 or 4.) I'm pretty sure we were up to "Dorothy IV" before we gave up on goldfish...