If your child has an average to below average size head, it can be very hard to find a helmet that fits. The helmet manufactures won't make helmets designed for children under 3 because they don't want the liability issues. So the smallest helmet you can buy is generally sized for a slightly less than average 3 year old's head.
Anyway, even for the correct size helmet getting the helmet pads and straps adjusted correctly for a good fit is very difficult with a 3 year old.
So for 1 year old to two year olds it is very challenging. The best option is to get the smallest helmet you can find and use extra thick pads or double layer the pads. It won't be really satisfactory, but it's really your only option, other than going helmetless - which regardless of the actual risks is socially unacceptable.
BTW - in Copenhagen no one wears helmets, not even infants http://l.yimg.com/g/images/spaceball.gif
Personally I know a lot of people that are glad it is hard to see in those trailers - because their kid isn't wearing a helmet.
this blog has a series of articles on family cycling http://bikeportland.org