What is the ruling on selling a sofa that is likely to have been contaminated with children's urine, given that there is no trace or smell of the urine, and is it sufficient to clean its visible surface?

If you are certain that the sofa has become impure, then it must be purified before selling it or informing the buyer of its impurity. If you are unsure whether the impurity reached its interior, the default assumption is that it did not, and purifying the exterior is sufficient. However, if you are certain that the impurity reached its interior, the method for purifying it is explained in Fatwa no. 116814. As for doubt regarding its impurity, nothing is required of you, because the default state is purity.