Suppose this Child template is a list item, and say, I have 1000 such list-items inside the parent, each with its own subscription - can Meteor/Mongo handle that??
If you are displaying only some data on the parent template and need the detail data when parent containers are clicked, then separating the detail subscription is wise.
But if you are creating a parent template that consists of childs and each child is to display the whole data the first time, than creating the monolithic subscription from the parent is a better choice.
If all the info is presented on the same page then it doesn’t really matter.
If the UI has tabs and navigation controls then I strongly recommend using a big subscription. Going a-la-carte seems like a good idea at first but you end up with a laggy user experience later. Every time the user moves to another tab your app has to go to the server to fetch the data.
What I do is subscribe to the minimum data needed on the parent template and then subscribe to the rest when the first batch is ready.