Hi Cazzybabs
I'd really recommend gps for young children; we have two. I think they are better than hamsters because they don't generally bite as much and they are not nocturnal, so the dc get to see more of them during the day. With careful handling when you first buy them, they become tame quite quickly and have lovely characters; I'm smitten anyway!!
(1) you don't have to have more than one but personally I think it's less cruel as they are herd animals ... I chose two who were already huddled up together in the pet shop. Two sows are best imo because (a) you don't obviously don't want them breeding and (b) because boars can get a problem with compacted faeces when they get older which you have to er, de-compact by hand(sorry if tmi!)
(2) mine live in our sitting room (!!) in a low-sided plastic bottomed cage which has a hay rack and a water bottle. The cage is open most of the day but they don't seem to want to make the effort to escape!! The cage should be adequately sized for two, but remember it should be easy to clean too, so personally I didn't buy the largest for that reason. In my view it's more important to have an adequately sized, easily cleaned cage where there's lots of stimulating things happening ..; rather than a huge cage that's difficult to clean that's sat in a shed away from activity ...
(3) gps have the same tolerance as us to weather so in the summer ours live out in a wooden ark on the lawn to graze during the day (I bring them in at night because of cats). An outside enclosure should always have a covered area to shelter them from sun/rain/marauding felines! Even now, they go out for an hour or so every day if the weather is OK.
As BeingPatient mentioned, they have very distinct characters: ours sing when we open the fridge and they squeak a warning to us if they need to get back to their cage for a "bathroom break" .
I can't imagine not coming home to a warm, inquisitive gp at the end of the day now ....mine actually falls asleep in my lap whilst I'm watching the telly ...and dd's likes to chase a willow ball around our wooden floor ..