Purpose: This single-case study examines a complexity approach to target selection in grammatical intervention in three children with varying levels of mastery of tense and agreement. Specifically, we examine whether targeting a complex tense and agreement grammatical structure (auxiliary BE in questions) leads to generalization to other less complex and related tense and agreement markers (auxiliary BE in declaratives, copula BE, third-person singular --s, and past tense --ed). Method: Three children (all boys; aged 5;5-9;7 [years;months]) with deficits in morphosyntax were enrolled in a treatment program targeting a complex grammatical structure (auxiliary BE in questions) following collection of multiple baselines. Children's performance on the complex structure and related tense and agreement markers were tracked before, during, and after the intervention across three different tasks. Results: Results show that, despite its grammatical complexity, the target was elicited in all three children with incomplete mastery of the tense and agreement system. Furthermore, all children demonstrated generalization to expressive language by increasing their mean length of utterance by approximately one morpheme during spontaneous language production following intervention. All children demonstrated individual patterns of generalization to other tense and agreement structures not targeted during intervention. Conclusions: These functional changes observed following intervention set the stage for future controlled studies to establish a stronger cause-effect relation. Taken together, this study contributes to an emerging body of work showing that complex grammatical targets may be used in intervention earlier than previously thought.