Overall, roslaunch won't provide it to you:

roslaunch does not provide any guarantees about what order nodes start in. This is intentional: there is no way to externally know when a node is fully initialized, so all code that is launched must be robust to launching in any order.

It highlights the correct approach to the implementation of robust code, that is easy to reuse later.

There are still some workarounds to it, mentioned in these questions:

• https://answers.ros.org/question/204487/order-of-nodes-in-roslaunch-for-accessing-camera-images/

• https://answers.ros.org/question/296982/how-to-handle-or-control-the-start-moment-of-nodes/