Wait until your little one reaches age one or sits up confidently without help....

https://gregorywads092.lowescouponn.com/can-i-use-a-child-bike-seat-on-a-mountain-bike-a-pro-s-guide-to-trail-ready-parenting

Wait until your little one reaches age one or sits up confidently without help. Strong neck muscles matter most because they must support their own head while wearing a helmet