You've got something chowing chewing big time..on the leaves of your little tree. I would clear a circle of 2' diameter around the base of your tree. Remove all weeds, as many weed roots possible without damaging the roots of this tree. Look at your other plants nearby to see if there is any of the same damage.
Make sure that the trunk of your baby tree is completely clear of the soil, mulch and weeds. There is a definite line between the bark of the trunk and where the roots begin. Make sure only the roots are under the soil.
As you are clearing, be looking for fat grubs, larvae, beetles and possibly a fat caterpillar that curls into a ball, cut worm. Record what you find and let us know. There is probably only one insect, a few individuals doing this damage. All other insects are fine.
This looks like a skeletonizer, leaf miner damage...possibly a beetle. You should find lots of insects in the soil and if you go out at night with a flash light you will find lots more that could be your culprit.
Leaf miners, skeletonizers are a huge class of insects, easily controlled but until we know for sure what insect it is no one should ever prescribe pesticides.
Most people plant their woody perennials too deep. This will cause them to be compromised, dying slowly and as such the plant becomes attractive to disease and insects. I think this is some sort of beetle, the skeletonizing is large, smaller skeletonizers actually leave more of the veination in the leaf.
Cut worm is one of the biggest easily noticed problems over where I've lived. And it only takes one or two to do big damage. You go out at night with a flashlight and you will find thick greenish caterpillars that when disturbed curl into a ball. Use scissors and cut them in two. Ugh. This is the best way to deal with most insects that are identified to be not welcome in the garden. They need to be identified, however.
If you find beetles, grubs, larvae, even cutworms please try to take a picture to send. Check with your local University Cooperative Extension Association or its equivalent over there. They will give you clues about your local environment and growing needs/problems that are priceless.