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Of course, when your child learns how to make Dua as a kid, they are more likely to continue performing Duas when adults. Also, this is a way to remind kids that everything occurs by the will and divine wisdom of Allah (SWT).
It might be hard to teach kids long Duas or Duas in Arabic (If non-Arabic speakers). The best Dua is the one that comes from the heart and going to extremes. Let the Dua be sincere and focused on the need in question.
The Prophet (PBUH) offered most of the duas that are connected to the prayer during the prayer, and he commanded us to offer them during it. This befits the state of the one who is praying, for he is facing his Lord and conversing with Him so long as he is praying.