Wait until your child is one. They must sit independently and support their...

https://wiki-coast.win/index.php/Rear-Mounted_Child_Bike_Seat_Age_Limit:_More_Than_Just_a_Number

Wait until your child is one. They must sit independently and support their head while wearing a helmet. Trailers provide the safest environment for infants, while front or rear seats suit active toddlers. Keep your first rides short and smooth.