Traditionally, in Wireless Sensor Networks, protocols are designed independently in the layered protocol stack, and metrics involved in several layers can be affected. Communication latency is one metric example, impacted by both the routing protocol in the network layer and the MAC protocol in the data link layer. Better performances can be obtained using cross-layer approaches.In this paper, we address latency optimizations for communications in sensor networks, based on cross-layer decisions. More particularly, we propose new time slot scheduling methods correlated to routing decisions. Slot allocation for nodes follows particular routing tree traversals, trying to reduce the gap between the slot of a child and that of its parent.Simulations show that latency performance of our contributions improves similar cross-layer approaches from 33 % up to 54 %. Duty cycle of obtained schedules are also improved from 7 % up to 11 %.