Focusing on the acquisition of questions in child second language acquisition (child SLA) German and French, this study investigates the matter of AO and sensitive period effects in the L2 acquisition of syntax and morphology. As such, it may shed light on cross-linguistic differences and similarities in children acquiring a second language in preschool. Two different groups of child second language learners are investigated, consisting of children with different typological language backgrounds (viz. Turkish and German) who learn typologically different second languages (viz. German and French, respectively). Our conclusion is threefold: (i) irrespective of the first language, child SLA is highly similar to first language acquisition (FLA) in the grammatical domain investigated; (ii) children with an age of onset of three to four form a heterogeneous group in terms of grammar development but may be considered simultaneous rather than successive bilinguals; and (iii) the input situation affects the interpretation of developmental patterns in child SLA. These aspects are discussed with reference to different claims about the characteristics of child SLA (i.e.Meisel 2008, 2009; Schwartz 2003a, b) and their consequences for linguistic modelling.