Functional communication training (FCT) is an evidence-based intervention that while often effective, can result in rote responding, reduced generalizability of target behavior, and resurgence of challenging behavior (CB) during treatment lapses. Lag schedules of reinforcement have been successfully used to address these concerns. We applied an increasing lag schedule within FCT to increase the variability and persistence of appropriate responding of four young children with disabilities while maintaining low levels of CB during treatment. Our results provide evidence regarding the effects of lag reinforcement on appropriate communication and CB during treatment and lapses in treatment with children with autism and Down syndrome. This research provides a new perspective to the field given, we assessed functional communication during baseline, assessed generalization to new contexts, and assessed social validity via both masked raters and participants’ mothers. We discuss the implications of this work and provide future directions for researcher and practice.