make-believe (n.)

"pretense, false or fanciful representation," 1811, from the verbal phrase make believe, which was used in children's talk by 1773 for "pretend;" see make (v.) + believe. As an adjective, "unreal, sham, pretended," by 1824. The noun form make-belief (1833) was "Substituted by some writers for MAKE-BELIEVE; the formation of the latter, being misunderstood, was imagined to be incorrect" [OED].

Let's-pretend (n.) in the same sense is attested by 1904 (the verbal phrase is from 1848). To make believe (v.) in the sense "cause to believe" is from late 14c.