The mother of a little boy who watched his best friend get killed by a four-wheel-drive as they rode their bikes has described the pair as "two peas in a pod".

Bianca Masi's eight-year-old son witnessed his friend Jake Beattie, also aged eight, struck by a car at The Ponds in Sydney's north-west on Monday night.

"It's still surreal," Mrs Masi told Fairfax Media.

"Every day they were together, they are like two peas in a pod, it's a bit hard trying to look after him and make sure he is ok," she said.