I would like to make a list of the *three* strongest arguments from a evolutionist and the same for a creationist as to why each thinks his/her point of view is correct about how life forms came into being and are the variety we see now.
I'll try to keep the language easy for the age group, but you can translate for them.
1) Whenever we find a proven or well evidenced cause for natural things, it is natural. So "natural causes" is always an infinitely more likely explanation for anything than any proposed alternative.
2) In our experience, chemical reactions make chemical phenomena (things), and life is a chemical phenomenon (thing).
3) The mechanisms (workings?) of the theory of evolution can be observed in action, and the theory of evolution fits the known evidence far too closely to be explained by coincidence (or thousands of coincidences).
Note: You asked for brevity, and for the strongest arguments, so all we can offer is the strongest brief arguments. The strongest arguments for the side with the evidence aren't really brief, because the more evidence listed, the stronger they get.
While you're at it, tell the kids that each of their cells contain scar patterns of ancestral viral attacks on their genes that match those in other simians so well that our descent from a common ancestral species with all monkeys and apes can be considered as much of an historical fact as the fact that the Normans invaded England in 1066.
I'm not exaggerating, but that last one doesn't really fit the way you phrased your request (how life came into being, and the variety).
As you're teaching RE, it's worth mentioning that there is nothing in the idea of the naturalistic origins of life and its variety that would exclude the idea of a god/creator of a universe in which such processes take place. That's another question.
You will be discussing all forms of creationism equally, and without cultural bias, I hope. It wouldn't be a good idea if the kids got the impression that any one of the below had more evidence to support it than the others. All zeros are equal.