Covert contrast is the statistically reliable distinction between target language phonemes produced in the process of language acquisition that is nevertheless not perceived by a native speaker of that language. This paper examines the acquisition of contrasts in four Najdi Arabic fricatives, /s/, /[voiceless palato-alveolar fricative]/, /[voiceless dental fricative]/, and /[voiced dental fricative]/, and seeks to identify the most common substitutions in producing these sounds. Words were elicited from 25 preschool children (aged 3-5 years). The target words contained the studied fricative followed by either long /a:/ or short /a/ in the initial position. Praat software was used for acoustic analysis to extract the four acoustic cues: center of gravity, fricative noise duration, F1, and F2. Participants performed a word repetition task and a picture elicitation task. The results showed six cases of covert contrast, seven cases of no contrast, and 87 cases of accurate production (overt contrast). The results also revealed stopping, gliding, and affrication as substitutions in the manner of articulation, while replacement in place of articulation occurred in backing but most commonly in fronting (24 cases). The present study sought to determine implications for children's linguistic performance, and, consequently, for language education and for planning treatment for children with speech disorders.