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

https://fair-wiki.win/index.php/Can_Twins_Ride_in_a_Bike_Trailer%3F_The_Mechanic%E2%80%99s_Guide_to_Family_Cycling

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