Two other children, aged 10 and seven, suffered minor injuries in the crash, in Ballards Road, off Church Lane, in Dagenham at 15:50 BST on Friday.
The 22-year-old motorcyclist  was taken to hospital where he later died.
Police said a KTM motorcycle was involved in a collision with a Vauxhall Zafira car. The 38-year-old male driver of the car was not injured.
A post-mortem examination is due to be carried out.
The boy, who was a rear passenger in the car, was taken to an east London hospital where he remains in a serious condition.
No arrests have been made and police are appealing for witnesses.
Anyone with information is asked to call the Serious Collision Investigation Unit based at Chadwell Heath.
A motorcyclist died and a five-year-old boy was left in a serious condition in a crash in east London.