Most pediatricians suggest waiting until your baby hits the 9 to 12-month mark....

https://astro-wiki.win/index.php/Can_I_carry_a_diaper_bag_while_using_a_rear_child_seat%3F

Most pediatricians suggest waiting until your baby hits the 9 to 12-month mark. Your child must hold their head up steadily while wearing a helmet and sit unsupported for several minutes. When they show this core strength, you have clear choices