Translation:The children sit out in the garden under the tree and play.
Many of these English translations are grammatically impossible, especially when it comes to motion verbs.
For a verb like "sit", English usually can't distinguish motion from state: "The children sit in the garden under the tree and play." or "The children sit out in the garden under the tree and play."
With an explicit motion verb: "The children go out and sit in the garden under the tree and play."
Likewise, in English, it's very unusual to say that a person "sits into a chair" (this is in reference to an earlier exercise). You can "get into a chair" or "sit down in a chair" or just "sit in a chair". But "sit" doesn't like to take prepositions of motion.
This answer is very awkward and it looks like a literal translation.
I'd rather say "The children are sitting under the tree in the garden and they are playing".
There is no need to even mention "out" - if you are in the garden you can't be inside.
The Hungarian sentence is looking at the entire action - the children going out of the house (ki-), into the garden (-be), sitting down under the tree (alá), and starting playing.