ABSTRACT A growing body of evidence suggests children acquire linguistic variation and follow grammatical constraints on its use from a young age. While prior research predominantly focuses on variables for which both variants are well-attested, stable variation in natural language often includes variants with an unbalanced distribution. In many cases, one variant is extremely rare, which poses a learning challenge for children and may require revision of accounts of children’s acquisition of variation. The negative morpheme ne in French is one such rare variant as it is only rarely realized (<10%) in adult speech. While previous research suggests clear constraints on ne-realization in interadult speech, less is known about ne in French children’s early production and input. In the current study, we analyze ne-realization in the conversational speech of 14 monolingual French-learning children (ages 0.10–8.01) and their caregivers. We found that, for both French children and their caregivers, ne-realization was rare and followed the same linguistic constraint found in interadult speech. Importantly, children showed probabilistic production following the constraint extremely early on – at age 2.05 on average. Further, we found a strong influence of input on variation acquisition: caregivers with higher ne-realization rates had children who used more ne and from a younger age. Taken together, our results suggested that young children can acquire statistically rare but systematically patterned variants, adding further evidence that they are skilled and sophisticated language learners.