Wait until your baby sits unassisted and holds their head steady, usually...

https://telegra.ph/Can-Two-Kids-Ride-on-One-Bike-with-Seats-Everything-You-Need-to-Know-05-10

Wait until your baby sits unassisted and holds their head steady, usually around nine to twelve months. Always prioritize neck strength over age. Before your first trip, ensure a toddler helmet fits snugly