This paper studies a time-varying version of the so-called containment problem for multi-agent systems. The proposed control strategy is decentralized and allows a subset of mobile agents (called leaders) to track a predetermined trajectory while they achieve a time-varying formation. Simultaneously, another subset of mobile agents (called followers) converge exponentially to the area bounded by the leaders. The results are presented for the front points of unicycle-type robots. The theoretical results are verified by numerical simulation.