A claim that Metropolitan Police officers used excessive force after stopping a car in west London is to be investigated by the Independent Police Complaints Commission (IPCC).
Three cousins, aged nine, 14 and 24, were pulled over near Hanger Lane as the eldest was using a mobile phone.
But the two older cousins said they suffered facial wounds, while the nine-year-old was allegedly handled roughly.
The IPCC said it would examine the methods and level of force used.
Territorial Support Group officers sounded the siren of an unmarked silver minibus and approached the men's car as they approached the A40, the IPCC said.
The trio alleged they were mistreated as the 24-year-old was given a fixed penalty notice for driving while on the phone.
They lodged a complaint with the Metropolitan Police a few hours after they were stopped and this was referred to the IPCC.
Investigators have been trying to trace CCTV from the local area and have made house-to-house enquiries in an attempt to find witnesses, the IPCC said.