A seat assembly comprises a booster seat with a lower support surface and an upper seating surface. A back support extends upwardly at the rear of the upper seating surface and side supports extend from the back support around the lateral edges of the seating surface to merge with the upper seating surface. A supplementary base has a peripheral edge to engage the periphery of the lower surface to provide increased lift. The base has an aperture dimensioned to receive the back support and side supports so that the base may be inverted and nested on the upper seating surface.

A convertible highchair is described that includes a bucket seat with a contoured backrest and an adjustable harness to securely and comfortably hold an infant in place while eating or performing various activities. The bucket seat, acting as an infant seat, is also removable to accommodate infants of different sizes and ages. For instance, the bucket seat can be removed to transition the highchair from use as a baby feeding chair to use as a toddler feeding chair, where a toddler seat is height adjustable. The highchair also features a removable tray that is adjustable in distance from the infant to allow for easy access to food and drinks. The tray can also be equipped with a spill-resistant lip to prevent food and drinks from spilling onto the infant's lap or the floor. The highchair also includes a sturdy base with legs that ensure stability on various surfaces.