A child safety seat assembly includes a shell body for defining a support base attachable to a child seat, a leg frame extendible downward from a bottom of the shell body, and a cushion part. The leg frame includes a first and a second tube segment attached with each other, the first tube segment being affixed with a fastener. The cushion part is affixed in the second tube segment adjacent to the fastener, and is formed as an integral hollow body including a first, a second and a third sidewall. The first sidewall faces the fastener, and the second and third sidewalls include inward recessed surfaces and are respectively connected with two side edges of the first sidewall. When the second tube segment is forced to displace relative to the first tube segment, the fastener and the first sidewall push against each other to cause deformation of the cushion part.