Continuity mistake: After Cliff talks to Peter, Theo is told to run to the neighbors across the street. However, he can be seen in the background of some shots when the twins do their game. As soon as the twins finish, Theo is shown coming back into the house.
Other mistake: During the scene where the twin sisters were singing their song, Theo was asked by Cliff to go to Peter's house and ask his mother if he could stay. Theo left immediately after this. But if you look closely while the twins are singing, you'll see Theo's arm in the scene. When the twins finish performing, they sit down. After this, you see Theo coming through the door after having just gone to Peter's house.
Chosen answer: That is the song "Open the Door Richard."