Bec - i had a similar issue, My son was always going to take my name without hessitation, but i had never been married and i was NOT in good terms with my father and did NOT want DS carring on my dads name so i changed my name to my mums maiden name which i loved being and then DS took that name. As im not married still this bub will also take this name so the kids are the same.

As someone said you can just make one up if u have issues with ur mums name - its the exact same process.