Correct Russian (simple):
Коала всю жизнь живёт на эвкалипте и ест его листья.
Another correct Russian (more formal):
Вся жизнь коалы проходит на эвкалипте, и она питается его листьями.
Correct English (if the koala lives in one tree):
Koala A spends its whole life in a eucalyptus tree, eating its leaves.
Correct English (if the koala uses many trees):
Koala A spends its whole life in eucalyptus trees, eating their leaves.
Why these corrections are better — explained simply:
- Russian: In your original sentence the actions (lives and eats) looked like they belonged to "вся жизнь" (the whole life). We must show that the koala is the one who lives and eats, so add "Коала" or "она".
- Russian: For younger readers you can use the simple verb "ест" (eats) instead of the more formal "питается".
- English: Start with a capital letter "Koala A". Use "its" for one tree (its leaves) or "their" for many trees (their leaves). The rest of the English sentence is fine.
If you want, I can make the sentences even shorter or turn them into a little story for a 7-year-old.