Wait until your baby sits up independently, usually around 9 to 12 months....

https://highstylife.com/can-i-use-a-child-bike-seat-on-an-e-bike-a-service-writers-guide-to-family-cycling/

Wait until your baby sits up independently, usually around 9 to 12 months. Always ensure they support their own head under a helmet. Front seats offer great interaction, rear seats feel secure, and trailers allow for comfy naps