The approximate number system (ANS) is foundational to numerical cognition. Whereas some studies hint at intact ANS skills in individuals with Down syndrome (DS), others report impaired ANS abilities. To reconcile these discrepant findings, we investigated ANS skills of children with DS and typically developing (TD) children in a non-symbolic number comparison task. Children had to indicate which of two characters possessed more dots. The two sets of dots differed by ratios of 1:2 to 4:5. While overall DS and TD participants' performance was comparable, accuracy levels differed for the two groups depending on the ratio presented. Both groups performed similarly for 'easier' ratios, however, TD children outperformed children with DS when they had to discriminate 'harder' ratios. Children with DS may thus face specific problems when discriminating ratios that are more demanding. The deviating ANS skills could potentially be related to problems with other symbolic maths abilities.