The Americans go all swivel eyed over this too..
We get many inquires from parents with babies aged about 6 weeks to 14 months about when it is safe to put a helmet on the child and take them for a bike ride. The question often reaches us as an inquiry on where to find an extremely small helmet.
Nobody we have met in the injury prevention field recommends taking an infant of less than 12 months in a bicycle child seat, trailer, sidecar or any other carrier. Nobody. And we do not either.
New York state law prohibits it. The US Consumer Product Safety Commission thinks it is dangerous to take a child under one year on a bicycle, and here is their rationale: Maurice Keenan, MD, from the American Academy of Pediatrics , requested that a minimum age of 1 year be reflected on the label for helmets intended for children under age 5. This would better convey the message that infants (children under age 1) should not be passengers on a bicycle under any circumstance.
The Commission agrees with the commenter that children under 1 year of age should not be on bicycles. Children are just learning to sit unsupported at about 9 months of age. Until this age, infants have not developed sufficient bone mass and muscle tone to enable them to sit unsupported with their backs straight. Pediatricians advise against having infants sitting in a slumped or curled position for prolonged periods. This position may even be exacerbated by the added weight of a bicycle helmet on the infant’s head. Because pediatricians recommend against having children under age 1 as passengers on bicycles, the Commission does not want the certification label to imply that children under age 1 can ride safely.
Source: 16 CFR Part 1203 Safety Standard for Bicycle Helmets; Final Rule, page 11726
That explains why you will not find a child helmet on the market sized for a tiny tot. You certainly do not want to ride with a bare-headed child, and in some places it is illegal. In fact, several states have laws against taking children under one year of age on a bicycle, even with a helmet.
Parents love their babies and love their bicycles, so it is natural to want to put the two together. That thought occurs to every bicycling parent, generally before the child is born. We see online postings by some parents who put their children in baby seats of one design or another and take them along on trailers starting as young as five weeks. Others use a baby backpack. At slightly older ages, people use front or rear-mounted child seats. A few (mostly in the UK) use sidecars. Each has its advantages and disadvantages.
A disclaimer: our purpose here is to highlight the risks. We are hearing more now about undiagnosed brain injuries, with symptoms too subtle for doctors to detect in a clinical setting, but very real to families. And that comes to mind whenever someone asks us about babies and biking. We worry about your six-week-old entering the first grade six years from now with a small but detectable mental handicap. That is alarmist; this is an alarmist page!
For an official US Government view, we have excerpts from the Consumer Product Safety Commission's age-related guidelines for ride-on toys."