A child bike seat cannot be chosen by the child’s age or the seat’s appearance alone. First establish where it will attach: to the handlebar stem, the rear luggage rack or the frame. Each option places different demands on the bicycle’s construction and permitted load.
For a rack-mounted seat, simply having a rack is not enough. Compare its stated load rating, width and tubing dimensions with the requirements of the specific seat. A frame-mounted seat instead depends on the shape, size and material of the seat tube; carbon and very lightweight frames are unsuitable for many such mounts.
Before buying, check the seat’s permitted child weight and the bicycle maker’s instructions. Once fitted, the seat should be firmly secured without wobble, and its attachments should be checked regularly. A sound choice starts with the actual bicycle, not the assumption that one seat fits them all.