It's recommended that you wait until your child is at least nine months old before taking them on family rides in a trailer. At this age, young passengers will have enough muscle tone to hold their head upright and stay in a seated position where there's no risk of injury, particularly to their neck.
Accessories such as slings and supports can be used for younger babies. But most bike trailer manufacturers recommend only using this additional equipment with your trailer in pushchair mode. If you still decide to use them in cycling mode, just make sure your child is wearing a helmet, even though they're in a trailer. That said, be aware that there are currently no helmets available that are approved for use by infants. The minimum available head size is 44 cm – the size of a 6- to 8-month-old's head.