My kids know both ways can happen & that doctors usually only need to get involved like that (c/S) if something is wrong.

I would be showing your own DD some nice gentle birth videos on You tube. This is what I did with my kids. They all know babies come out of your vagina but also know that sometimes they (mum & or bub) need help & the quickest way to do that is an operation.

Tell your nephew & niece to talk to their Mum about it.