A small, ordinary wrong
Madyan were traders, and their wrong was not dramatic. They measured generously when they were buying and meanly when they were selling. The Quran says: Give full measure and weight in justice, and do not deprive the people of their due (11:85).
This is a good story to tell precisely because the sin is small and boring. There is no idol, no monument, no open outrage. There is a thumb on a scale.
Shu'ayb connects it directly to belief: he opens with worship Allah, you have no deity other than Him — and then goes straight to the measuring, in the same breath (11:84). The Quran does not treat honest trading as a separate matter from faith.
What they said back
Their answer is worth reading to a child exactly as it stands: O Shu'ayb, does your prayer command you that we should leave what our fathers worship, or that we should do with our wealth what we please? (11:87).
It is sarcastic, and it is the oldest objection there is — keep your religion out of my business. Children hear versions of that argument, and it helps them to know the Quran recorded it and then answered it.
The line about himself
Shu'ayb's reply contains the sentence his story is remembered for: And I do not intend to differ from you in that which I have forbidden you. I only intend reform as much as I am able (11:88).
He is saying: I am not telling you to do something I exempt myself from. For a child, that is the most portable idea in the account — the test of whether someone means what they are telling you is whether they do it themselves.
He also says something that softens the whole passage: And my success is not but through Allah. Upon Him I have relied, and to Him I return (11:88). Then, when they refused, the account closes as the others do — those who wronged were seized, and Shu'ayb and those who believed with him were saved (11:94).