From the article:

After a video of parts of the play did the rounds on social media, the accusation was that a child spoke a line in the play that Narendra Modi should be beaten with slippers. However, several media reports have debunked this claim. The child seems to have made a generic statement that anyone who demands documents to prove one’s citizenship should be beaten with chappals.

It's absolutely wrong to detain the kid for this. Only the one who wrote the lines for "beating" someone deserves to be questioned and that too in accordance with the law.