My kid says we have to have some music for her routine and it has to be exactly X minutes long. How do I do that?

Well, your coach may be able to help out here. Many experienced coaches have a small library of music that they can use. If you want to do it yourself, you probably have to start with a piece of instrumental music (no vocals).

Then, you'll need a computer and some editing software to cut the piece down to size. I have lots of software that does this kind of stuff but there is a good free software called Audacity which is fairly easy to use. Even with mastery of the software, it takes a bit of work to get a smooth edit and some songs just don't lend themselves to certain lengths. It gets easier when you get up to two and three minute songs.

Usually a competition publishes the requirements for the specific length of song that is allowed/required for each level of competition. Usually there is a + or - 10 seconds allowed. So if they say that your song has to be 1.5 minutes, your song could be anywhere from 1:20 to 1:40 but make sure you have it right for the competition you're attending. This information is usually published with the announcement of the competition.