Just got rockband for ps3 and tried to hook it up so my kids could play and the drums and guitar wont play, just the microphone. what is wrong or what else could i do?

just got rockband for ps3 for my kids for christmas. tried to hook it up for them to play it and the microphone is the only thing that works it seems. the guitar and the drums light up but they will not join in to play like a band. what else could i try to make this thing work. its their first time owning a ps3