These are cheaper than trailers and require less leg and lung power. They're great for outings in better weather. Most suit children from nine months to three or four years; a weight limit of 18-20kg is usual. After fitting the seat, accustom yourself to the bike's compromised handling by taking a trial run with a big sack of spuds. Practise getting your leg over the top-tube without swinging it over the saddle, or you'll kick your passenger in the head!
Front seats are less popular but affect the handling less than rear seats. However they can force you to ride bow-legged - okay for shorter distances but irritating on a longer ride. When fitting the seat, ensure the base of the seat back is above or in front of the rear axle. Weight further back can make handling very difficult. Better control is possible with wide bars, flat or riser.
We recommend that whatever seat you choose, get an additional bike attachment for swapping between bikes.