The majority of toddler bike seats will mount to an adult bicycle behind the saddle and over the rear wheel, though some models instead mount near the handlebars of the bicycle. Both models have advantages and disadvantages concerning ease of use as well as safety, and each style can have features to enhance the safety and convenience of the seat. A third option for toddler bike seats that is quite a bit safer but also more expensive is the toddler bike trailer, which attaches to the bike's rear axle and trails behind the bike on its own set of wheels.
Rear-mounted toddler bike seats are perhaps the most common because they are generally easy to install and they will not interfere with the adult's operating of the bicycle. When choosing such a seat, be sure to choose a model that secures properly to the bicycle at more than one point; this ensures the seat will stay in place should one point of contact fail. Any toddler bike seats that mount in this position should have leg compartments for the child to prevent dangling legs from being near the rotating wheel, and all seat models should feature a shoulder harness and/or lap harness to secure the child into the seat.
Toddler bike seats that mount in front of the adult rider will secure to the top tube of the bicycle. This type of seat tends to be less safe than rear-mounted models because the adult rider must hold the handlebars around the child, which can lead to instability while riding. If you are considering this type of seat, only use it for small children, and try to choose a model that is not exceptionally wide so your movement of the handlebar is not limited too severely.
A somewhat newer option that is much safer and often more comfortable for the child is the toddler bike trailer. These units do tend to be heavier and more expensive, but they are convenient and safe both for the child and the adult rider. The trailer features its own set of tires, a comfortable cockpit for the child, and an arm that extends to the adult's bicycle. The arm secures to the seatpost or rear part of the frame. One of the biggest advantages of this type of seat is the fact that if the adult falls off his or her bike, the child will not fall with the adult rider from a high position. The trailer may tip over, but the child will not fall as far and may not tip over at all.