We examined children's productions of mainstream and non-mainstream relative clause markers (e.g. "that", "who", "which", "what", "where", [image omitted]) in African American English (AAE) and Southern White English (SWE) as a function of three linguistic variables (syntactic role of the marker, humanness of the antecedent and adjacency of the noun phrase head). The data were language samples from 99 typically developing 4-6-year-olds: 61 spoke AAE and 38 spoke SWE. The majority of the children's relative clauses included mainstream markers. Non-mainstream markers were rare, with 3-6% involving [image omitted] subjects and 2% involving "what". The children produced who exclusively as subjects and with human antecedents, "where" exclusively as locatives and with non-human antecedents and [image omitted] and "what" primarily as direct objects or objects of prepositions and with non-human antecedents. Although AAE- and SWE-speaking children produce some non-mainstream relative markers, the majority of their markers are mainstream. Their use of relative markers is also influenced by linguistic variables in ways that are consistent with a wide range of mainstream and non-mainstream English dialects. These findings show across-dialect similarity in children's relative clauses, even though characterisation of relative clauses as a contrastive dialect structure remains justified. (Contains 4 tables and 3 notes.)