The single-vehicle crash took place on the M4 eastbound junction 10 exit slip road onto the A329M, near Wokingham, on Sunday afternoon, Thames Valley Police said.
In a post on X, the force said: “Sadly, a child died in the collision and our thoughts remain with the family who are being supported at this extremely difficult time.
“Other passengers sustained serious injuries, requiring hospital treatment. We would ask motorists to continue to avoid the area.”
The incident happened shortly after 2.45pm, National Highways said.
The motorway was closed in both directions between junctions 11 and 8 for several hours, and the slip road remains closed on Sunday night, with officers still at the scene.
No arrests have been made, the force said.
