They started Lisa's party by having the kids, aside from Lisa, perform something.
Ashlee and the others began singing songs that all of them knew. So, all the songs they sang were short children's songs.
It was also obvious that they hadn't practiced much because, while singing, some actually forgot what came next and just followed the words of the other kids, especially Sophie.
Their hearing was already very enhanced, so the kids' mistakes were very obvious to them, but no one bothered mentioning it. They just cheered the kids on and clapped after the kids finished singing.
In the last two songs, Lisa also accompanied the other kids in singing.
After Lisa and the others, the older kids also went to perform. They obviously lacked time, but there were a lot of famous songs before the apocalypse began, so even though they hadn't practiced anything, they still went on stage and sang together.
After they sang two songs together, they also began performing individually.
