in order to obtain data from an associated object you will need to retrieve it first. You can only access objects that are retrieved as part of your microflow (or alternatively passed as input parameter, or objects that are returned by a submicroflow).
In other words, you should add a retrieve object action (by association). Based on your domain model, the output will be a list of tasks as there can be multiple childs.
Depending on your usecase, you can either:
add a loop activity iterating over the list in order to obtain data from individual Tasks objects
2) If you are sure that there is only 1 “Tasks” object returned by the REST call, you could also add a "list operation (Head)" microflow activity in order to retrieve the first object in the list and access the data from there
Hope this helps!