Wait until your baby reaches twelve months to ensure their neck muscles support...

https://mega-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 twelve months to ensure their neck muscles support a helmet. Look for independent sitting before riding. Choose front-mounted seats for engagement, rear seats for stability, or trailers for the safest, comfiest nap-time rides.