Object-oriented – C++ Virtual destructors used only when there are virtual functions

First things first, this can be a code smell. The point of using composition for the parent/children is that the parent knows about the children but not vice versa. Especially if the relation is more of a 'contains' than 'is composed of'.

A reference to the parent is possible and fairly common in C++. In other languages, a function object or event is more often used to allow the child to communicate things that outsiders might want to know. This is a common publish-subscriber sort of pattern. I suspect that which is more idiomatic depends on which version of C++ you're using and the standards of your codebase.

If the responsibility for cleaning up dynamically allocated stuff shifts between your wrapper class and the C library based on how things get used, you are either dealing with a badly designed C library, or you are trying to do too much in your wrapper class.

In the first case, all you can do is keep track of who is responsible for the cleanup and hope no mistakes are made (either by you or by the maintainers of the C library).

In the second case, you should rethink your wrapper design. Does all the functionality belong together in the same class, or can it be split into multiple classes. Perhaps the C library uses something similar to the Facade design pattern and you should keep a similar structure in your C++ wrapper.

In any case, even if the C library is responsible for cleaning up some stuff, there is nothing wrong with keeping a reference/pointer to that stuff. You just need to remember that you are not responsible for cleaning up what the pointer refers to.