Does young bilingual children's code-mixing obey the same structural constraints as bilingual adults' code-mixing? The present study addresses this question using code-mixing data from 15 French–English bilingual children filmed in conversation with both parents at six-month intervals from the age of 2;0 to 3;6. The children's code-mixed utterances were examined for violations of the principles set out in the Matrix-Language Frame model (e.g. Myers-Scotton, 1993, 1997). The results show that the children obeyed all the constraints set out in the Matrix Language Frame model the majority of the time. With respect to the Morpheme Order Principle and to the interaction of Congruence and Matrix Language Blocking, they demonstrated consistent adherence with only marginal violations from the outset. In contrast, the children produced comparatively more frequent violations of the System Morpheme Principle and showed increasing adherence to this principle over time. We discuss possible explanations for the contrast between the children's performance on the System Morpheme Principle and the other constraints, which include the unequal emergence of INFL in the acquisition of French and English.