Wait until your baby hits their first birthday or sits confidently on their own...

https://web-wiki.win/index.php/What_Age_Can_a_Toddler_Ride_in_a_Rear_Bike_Seat%3F_A_Mechanic%E2%80%99s_Guide_to_Family_Cycling

Wait until your baby hits their first birthday or sits confidently on their own before you ride together. Your little co-pilot needs strong neck muscles to hold their head up in a helmet. Front-mounted seats offer a great view and keep them close