I go with Tom but then TDD is pretty much second nature to me.
If you don't have this or simply want to do it another way then:
Make an assembly surface of the outside of the parent tube then use this to cut your child part. Never tried to make the cut in assembly but you could definitely make the surface of the parent in the child while in assembly and do the cut in the child. Definitely creates cross references and these can get very tangled up if you are not careful (hence my preference for the formality of TDD). Also there can be an issue of cross referencing not being allowed in your setup (by system admin). You are allowed cross references by default in ProE/Creo but configs can be set to limit or disallow such references.
Do the same thing but activate the child and Copy the outer surfaces from the parent then use this quilt to cut away the unwanted "intersection". This is probably what I would do if I did not have TDD.
You can hunt and peck at cutting away the "intersection" as made up features in the child but there is no guarantee that if you update the model to say 50° then the "intersection" will no longer be valid and must also be manually updated. Do not recommend this as it is a minefield for later.
Regards, Brent