Is the couch considered impure if a child up to 13 years old sat on it without underwear, knowing that I did not see him urinate on it?

It is impermissible to look at the 'awrah (private parts) of a thirteen-year-old boy, which is from his navel to his knee, according to the consensus of the four madhhabs (schools of thought). For the Shafi'is, the 'awrah of a boy is the same as that of an adult. For the Hanafis and Hanbalis, a boy who has reached the age of ten has an 'awrah similar to that of an adult. As for the Malikis, a boy aged thirteen or more has an 'awrah like that of men. Attention must be paid to this matter, and there should be no leniency in uncovering the 'awrah of anyone who has reached this age.