Adults omit particles all the time too, in casual speech. It's probably better, however, to use all the particles until you've heard enough casual speech to know where it's acceptable to omit them. I feel like in this case, わたし、おんがくがだいすきです is fine, but I'm not a native speaker so my 'feeling' could be wrong.
It's also good to pay attention to where わたし(は) can be omitted; in many case you don't need to specify yourself. It's often understood, especially in expressions of tastes, desires, and opinions, that the speaker is the unstated topic or subject.