Tezza - yes, great. I found doing it with Year 1s though that the best stage was to let them actually do it. So heap up a massive pile of skins, feathers, snake sheds etc (as well as using real animals, just not in the pile!) and let them physically feel them all and then sort them out into different points in the classroom. If they then ask about dolphins etc, great, and you can mention it. But at that stage, I am just happy with them getting to grips with the basic principle - exceptions can come later (lie your Year 3s.)
Incidentally, it can get very complicated.
All mammals feed young on milk.
So what about doves and pigeons? They feed their young on the lining of the throat which they regurgitate - it's called crop milk. Does that make them mammals? No, clearly not. So is it that mammals feed on milk by the mammary glands? Because the monotremes don't have mammary glands - they ooze it from a bald patch near their groin. So are they now not mammals?
Ultimately classification is not a clear cut topic because the living world is not a clear cut place. Classification is an artificial attempt by us to make some sense of the world around us.
Try defining the word species, for example. We all know what a species is but there isn't a good definition that covers all options. So do we say that to kids? Nope. We give them a broad definition which is broadly accurate and then gradually introduce the anomalies to ponder on.
Same with classification. These big artificial lines we want so badly to be there, in the main, just aren't. They are in our heads. But there are things which annoyingly don't fit the patterns.
So let's discuss the broad patterns, because by and large the vast majority of mammals do have fur and do produce live young and do feed babies milk. And then let the kids process that idea, and get a handle on the whole concept, including why we do it, and make some sense out of it. And then if they are ahead enough to ask, or if they have pretty much got their heads around it, introduce the animals which don't fit nicely into the box, or provide information based on things they can't easily see or imagine.
Patterns are important even if there are exceptions but if you start with 'this is mostly true but this animal doesn't fit and neither does this one or this one' the kids will start to feel like nothing fits so why even bother!