Why is Mah Nishtanah recited by the children?

No, not in order to arouse their curiosity. The origin of the practice is probably.... a mistake.

In many early versions of the Haggadah, it is clear that the father asked Mah Nishtanah, including in the writings of Maimonides and in Mahzor Vitry, written by a student of Rashi's in 11th century France. 

There is also some evidence from the Talmud itself. In Psachim 10:4, the mishnah explains that after the second cup, the son should start asking his own questions about the laws of Passover and the father should answer them directly. If the son doesn't do so, however, the father must "teach him" - and the example brought of what he might say is "Mah Nishtanah". Hence, on the previous page, Rabba tells a young pupil who asks a sophisticated question, "Now you have removed my obligation to say 'Mah Nishtanah'.

So how did a parallel tradition of the son asking Mah Nishtanah develop, and become universal? 

Drawing on the mishnah above, Psachim 116a discusses who asks the man of the house questions about the laws of Passover. If his son is smart, it says, this is his role. If he is not smart, his wife can ask. And if not, he asks himself.  Even two wise scholars who know all about the laws of Passover, it adds, "must ask each other".

Immediately after this, the Gemarah quotes the Mah Nishtanah and then starts commenting on it. Daniel Goldschmidt surmises that at some point in history, someone mistakenly connected the line of Mah Nishtanah to the previous few lines, and understood that it was preferable for the son to ask about Mah Nishtanah.

haggadah Section: -- Four Questions
Source: Goldschmidt