Depending upon what you're writing the form in might determine the easiest method for calculating a "Date + 7" value per child.
Regardless, in Flow you can take a String variable that holds a date and use the addDays(stringDate, numberOfDays) function to add seven days. So if you received a string Date/Time as an input (or pulled by a query), then using this function will do it for you.
If I were doing the Form in PowerApps, then I personally would probably do this inside the application. However, for iterating through a list of child items in Flow where the data is already written to SharePoint, then you can use the Initialize Variable step to save the original task's date, use the SharePoint GetItems step to grab all of the children, then do an ApplyToEach step to loop through each child, use the Set Variable step to add 7 to the original date, then the Update Item step to write the new value for the child's date.
Italics below are the values you would change in each step to reflect your own list:
- Initialize Variable| taskDate: parentDate
- Get Items | Filter Query: parentID eq firstTaskID | Order By: childOrder asc
- Apply To Each| Value
- Set Variable| taskDate:addDays(taskDate,7)
- Update Item(completeTaskByDate: taskDate)
I'd still likely approach this at the Form (if possible), but this could be done pretty easily post-write.
