Wait until your little one reaches nine to twelve months before riding...

https://papa-wiki.win/index.php/How_Tight_Should_the_Harness_Be_in_a_Child_Bike_Seat%3F_The_Master_Guide

Wait until your little one reaches nine to twelve months before riding together. You need to see strong neck control and the ability to sit up completely unsupported. If their head wobbles in a lightweight helmet, hold off a bit longer