They'll fit in somehow - it might stunt their growth and they'll moan a bit, but I'm sure you'll manage to cram them in...
Seriously, it will depend to a large extent how tall you are! I'm a real shortarse, so I don't have the front seat very far back which leaves a bit of leg-room in the back. Although, even then, it’s not really a lot!
There's probably enough room for a couple of children, I don't really know, I've never tried to fit any in!
I’m not sure it would be particularly comfortable to sit there for any length of time though.