The system of relative clauses is considered as a complex system. One of the implications of this statement ought to be that its acquisition should pose difficulties for children. An identical experimental protocol has been implemented with French children (aged 4-10 years) and German children (aged 6-10 years). It consists in asking these children to retell individually the story which has just been read to them. The study shows that out of the six relative clauses contained in the source text, only two non-defining relative clauses, with a significant diegetic weight, have been relatively often reformulated by a relative, although only by French children. The occurrence of a relative pronoun is not a sufficient criterion to evaluate the progress in the acquisition of the system of relative clauses. In contrast, the paraphrastic reformulation of complex phenomena which are attested in the original relative clauses (as opposed to their suppression, semantic change and repetition), provides a reliable criterion for the level of acquisition of relative clauses.