Narrated Ibn 'Abbas: from 'Ubayy bin Ka'b, that the Prophet ﷺ would recite: "Qad Balaghta Min Ladunni 'Udhra" (18:76) with heaviness (Muthaqqalah - meaning with Tashdid on the Nun in "Ladunni")".
The lesson here for children is to be mindful when reading or reciting verses from the Quran; they should pay attention to every word they are saying so that they can understand its true meaning and appreciate its beauty. They should also remember to show respect for Allah's words by taking their time while reading or reciting them.