Most experts suggest waiting until your little one reaches nine to twelve...
https://mega-wiki.win/index.php/Can_Twins_Ride_in_a_Bike_Trailer%3F_The_Mechanic%E2%80%99s_Guide_to_Family_Cycling
Most experts suggest waiting until your little one reaches nine to twelve months before hitting the trails. Your baby needs solid neck strength to support a helmet and sit upright for the duration of your ride