"A boy plays and falls."
Translation:Knabo ludas kaj falas.
Because it's not correct. The simple answer is it's not grammatical, which you can tell without even knowing the vocabulary. In the English, "plays" is a verb, but in your translation you are using a noun, since any word ending in -o (or -on, -oj, -ojn) is a noun.
More specifically though, teatro is a theater, and teatraĵo is a "theater thing" or a play/theatrical production, so teatraĵojn is theatrical plays as a direct object of a verb; and the only verb you had was falas.
"iu" is "some". Not as in "some (random) boy", which I think is idiomatic English, but as in the partitive. Since "boy" is a singular discrete noun, that wouldn't be grammatical. Also, Esperanto does not have the equivalent to "a/an", so just "knabo" would be "a boy" or "boy" depending on what is appropriate for the English translation.