"Willem en Saskia zitten in een boom..."
Translation:Willem and Saskia are sitting in a tree...
I guess I cannot say about Australia, but in Canada at least, both statement are valid, but they mean different things.
"Sitting in a tree" is where a person sits on say a branch, of a large tree.
"Sitting on a tree" would be where a person sits on a small sapling or other small tree, where they sit on the entire tree, or most of it.
"Sitting in a tree" is a much more common thing to happen, but the other is still possible to express.
I assume that each are both expressible in Dutch as well, "op een tree" would be "on a tree", but I do not know for sure.