"Do you have children's music?"
Translation:У вас есть детская музыка?
I guessed as much. "We" is a rather poor defined term. Still, I am not sure how many native speakers agree with you. The expressions both mean the same but I would not use «музыка для детей» other than in the title of a book, maybe. The corpus search, too, suggests than "музыка для детей" is seldom used.
It is just that I saw you message in a different topic where you seem to suggest that natives use "рисовая каша с молоком", which is just a word combination you might use — for one, I did not even know such meal. Абрикосовое рагу с огурцами is no worse.
What I mean, a native speaker might say that occasionally, which is not the same as "we prefer" or "we call it that".
why is У тебя детская музыка marked as wrong? do we have to use plural/formal form instead of singular in this context? since English does not distinguish between 2nd person singular and plural it is pretty tricky to interpret "you" to russian equivalent. in other lesson tough, both are accepted (ты and вы). there is still unfinished work in this lesson maybe?