If your API is based on IDispatch you could use the dynamic keyword, something like this:

dynamic childObject = GetChildObjectSomehow();
childObject.CallAnyMethod() // compile will always succeed, will be resolved at runtime (and failed if there's like a typo error)

Note dynamic is not available on .NET core for COM objects yet.

If it's not (if it's based on IUnknown), then you'll have to declare this interface either in an external dll or .tlb, or directly in your C# code, so it can be called by the .NET runtime. You don't have to use the original .dll, you can redefine the interface by yourself if needed (maybe a simplfied version). But the runtime has to know the binary layout to be able to call it.