A convertible highchair assembly includes a seat frame, a booster seat detachably installable on the seat frame, a tray detachably mountable on the booster seat and having a plurality of sockets, and a plurality of leg extensions selectively attachable to any of the support frame portion and the tray, each of the leg extensions having a connecting end. The seat frame includes a support frame portion and a seat portion affixed with each other, the seat portion being adapted to receive a child. The connecting ends of the leg extensions are attached to the support frame portion for configuring a highchair, and the connecting ends of the leg extensions are respectively inserted into the sockets while the tray is removed from the booster seat for configuring a standalone table, the leg extensions providing standing support for the tray.