5- and 6-year-old monolingual (English) and heritage language (English–Spanish, English–Urdu) children completed a grammaticality judgment test to evaluate their awareness of grammatically right and wrong morphosyntactic structures in English. Results showed that language group differences were reduced when heritage language children displayed average receptive vocabulary competence for the sample, and when more difficult morphosyntactic structures were evaluated. In this middle range, sole two group differences were found. The findings underline the need to look at factors like receptive vocabulary when evaluating morphosyntactic awareness and language group differences.