Factual error: The route the children take to their house on the moped doesn't make sense - this is just to show the London landmarks.
Continuity mistake: After Don escapes from the house at the beginning of the film, the infected are right behind him. He manages to escape in the boat but when the following shots show the river, there is no sign of any bodies or blood (despite one infected being killed by the boats propeller) or any disturbances on the surface apart from waves being caused by the boat. There is no chance that the infected could have all gotten out of the river so quick (including the dead infected body) without leaving so much as a splash. (00:10:35)
Answer: Nobody ever said they could only come out at night. As their food became scarce, they'd taken to daylight hunting as well.
Captain Defenestrator