Wait until your baby reaches nine to twelve months. They need strong neck...

https://wiki-stock.win/index.php/Can_a_Baby_Ride_in_a_Bike_Seat_Without_Sitting_Up_Fully%3F

Wait until your baby reaches nine to twelve months. They need strong neck muscles to support a helmet while you ride. If your child sits unassisted and holds their head steady, they are ready for their first outing