Hi, and welcome!
Firstly, let me say: good choice! The Stagea is a great car, and you will be more than happy if you decide to buy one.
As far as child seats go, have a look here. There are plenty of owners amongst our group who have kids and use child seats in them. Some of us even have THREE child seats in the back of theirs.
I'm assuming your '94 Camry is one of the "wide body" versions, and the Stagea is roughly the same size as that, maybe slightly larger. It's not quite Commodore or Falcon sized, but it is a much better car than those two in so many ways.
Fuel consumption is not really a highlight, unfortunately, as they're a heavy car with a turbocharged engine, but they can be quite good if you drive them smoothly and mostly on a light throttle. It might be slightly higher than what you've quoted, but that figure is achievable. The RWD versions are roughly 100-150kg lighter, so you *might* see some improvements there.
If you have any further questions, then do a quick search within this area of SAU - most topics have been covered at least once or twice each. If you're still unsure, then feel free to ask away!