When I link an opportunity to a parent opportunity, how do I see it (and others) within the parent opportunity?

Opportunities do not have a native relationship back to themselves - so I'm guessing you're looking at a custom relationship/relate field that has been added to your system.

If it is a custom many-to-one relationship, you're selecting the parent opportunity on the record view, and then if you click to the parent and scroll down to the list of subpanels, there should be a subpanel called "Opportunities" - showing you each of the child opportunities.

If you do not see that subpanel, it could mean:

1. the subpanel is hidden  - which is easy to check by seeing if you can see this subpanel on the Accounts page - if it doesn't show there, you can show them again using the instructions here: https://support.sugarcrm.com/Documentation/Sugar_Versions/10.1/Sell/Administration_Guide/Developer_Tools/#Display_Modules_and_Subpanels
2. either a relate field or a 1:1 relationship was used instead of a many-to-one relationship. You cannot change this - you may as well either delete the field/relationship, or just hide it. Worth reading this to understand how they work:https://support.sugarcrm.com/Knowledge_Base/Studio_and_Module_Builder/Introduction_to_Relationships_and_Relate_Fields/#Relate_Fields 

Hopefully that addresses your question!

Obviously, I don't know anything about your business/context, but something to be mindful of: linking opportunities to a parent opp can have some interesting impacts upon your pipeline - you need to be careful you're not double counting revenue. Although there are plenty of good reasons to link opportunities together (e.g. they all relate to the same project), I would generally not use a parent opportunity to do so, as the risk of reporting potential revenue incorrectly is too high. I'd group it by some other record instead, based upon the given business need.