Simples. Keep Zed for when it's just you or weekend car, buy a replacement for your girlfriend's car. She can drive the mpv/ 5 door hatchback.
How often would you need to go out just you and your little boy? With the other car not being available as she's gone out? Keep it simple, when she's out, stay at home. Once he's old enough he'd love it.
PS congrats!!!