쯔아식 mimics how some people say 자식 in a drawn out way. It is similar to spelling "please" as "Puleeeze!" in English. 자식 is most commonly is a noun for "a son or daughter", i.e. a child of someone. 어둠의 자식들 (The children of the darkness) is a movie title, for example. It is also a somewhat derogatory noun or address word used for a male. 이 자식아! is like "You bastard!" said in anger. If used by itself with the right intonation, it is an endearing exclamation. You say 자식!, or its variation 짜식!, 짜아식! (or even 쯔아식! as in your example), etc. by itself like an interjection, similar to "(You're / this is) some boy/guy!". This usually carries a tone of endearment or mild amazement because the person (usually younger than the speaker) did something unexpectedly cool. For example, 자식, 벌써 다 컸네 is like "Look at him. He's now all grown.".