I don't have a mini but have been in the back of one (a 4 seater one) as a reasonably tall (5'9") adult and I've seen someone in one of the bigger 5 seat ones pick up 4 teenagers from high school. I'd definitely go 4/5 doors though as I ended up having to get rid of my 3 door car when mine became teenagers as it was a pain them getting in and out.
I have a 5 door mini Countryman. It's pretty big inside but easy to park. Boot is big enough for maclaren stroller. The controls light up in rainbow colours 😍🌈 I've driven lots of BMW s and the engine and controls are basically the same. Highly recommended. There's nothing I don't like about it!
I have a 3 door mini and there's no room at all in the back, it's really difficult to get in and out of without launching yourself. My mum, however, has the 5 door hatch version and hers is loads more roomy in the back so definitely possible with 2 DC