Wait until your baby is 9–12 months old and sits independently with strong neck...

https://list-wiki.win/index.php/How_to_Descend_Hills_Safely_with_a_Child_on_Your_Bike

Wait until your baby is 9–12 months old and sits independently with strong neck control. Front seats allow for interaction, rear seats offer stability, and trailers handle rougher terrain best. Always prioritize a perfect helmet fit for every ride