This article reports a research on late phonological assimilation processes in child language in order to determine the possible stages in their evolution and the variations in relative frequency as a function of directionality (progressive vs. regressive), distance (contiguous vs. non-contiguous), and type of phonemes involved in assimilations (consonants vs. vowels). The corpus analyzed was collected from 240 children ranging in age between three and six years old. The speech samples were obtained in naturalistic settings of spontaneous conversation with the researchers, and were transcribed and analyzed with the tools provided by the CHILDES Project. Results describe a profile with three stages, reflecting the general dynamics of phonological development. Around the age of four, a general reorganization of the phonological system takes place, and the least frequent assimilation processes (progressive, contiguous and intervocalic) tend to disappear earlier. The most frequent assimilation processes (regressive, non-contiguous and inter-consonant) tend to persist as processing errors in the transition to adult speech. These findings show the functional complexity of late assimilation processes, as well as the need for taking it into account when planning clinical intervention.