Were you able to find a way to get your child a music pass account? I'm asking because I just ran into a similar problem last weekend. I was thinking of getting my son an Xbox Music pass, but before doing that I thought I'd show him how to take advantage of the free and reduced price album deals Microsoft has been running. He has a Microsoft/Xbox Live Gold account with some money in it, and I figured it would be no problem. Nope. Turns out child and teen accounts can't purchase music, at least not in the Windows 8 app. You have to be 18 to buy music - not explicit music, any music. Unbelievable.
However, on the Microsoft community forum, the mod said while he couldn't purchase music, he should be able to redeem a code for a music pass on his account. I haven't tried that yet; despite the mods assurances, I wasn't convinced it would work. Your experience isn't doing anything for my confidence in that solution either...