Twin chorus

The two girls are adorable. Dressed alike, with pink outfits and pink shoes. They have long black straight hair, in a flowing coiffure slightly out-of place for their age. They must be around 4 or 5 and they have trouble keeping up with their mother who's walking fast, holding her daughters on her side. The mother is dressed all in black with a large grey backpack over her shoulders. I am walking behind them on my way back from the subway. Even I have trouble in keeping up with the mother's pace. The girls are practically running on her side.
I hear her speaking Spanish, the girls answer in English. "I'm tired" says the one on her right. "I'm sick" says the one on her left. "I'm tired" , "I'm sick", "I'm tired", "I'm sick". Very quickly this evolves into a song. The kids are chanting it with obvious delight: "I'm sick", "I'm tired", "I'm sick","I'm tired". The mother is still running.