Why do you want to keep your dogs outside at night, jmg1? Is there any reason for it (e.g. to stop them weeing in the house or something)? Or is it just because it's what everyone else does?
I'm sure your dogs would get used to living outside, as long as you make sure they are nice and warm. For a while, I lived in a shared house on a farm, where the farm dogs all lived outside at night. The farmer didn't feel that it was fair for my dog to stay in while his went out at night, so we literally filled the kennel with straw (because my little dog didn't have a very thick coat and was used to being in a centrally heated house) and within a few days he loved it. He used to stand by the door at 10.00 p.m. waiting to go to bed! He was about 6 or 7 at the time, so I don't see a problem for your 8-year-olds. Would your dogs have access to some outside space (e.g. a run) if they needed a wee in the night? Personally, I wouldn't be happy leaving them totally shut up. Nor would I let them have the freedom to roam around the whole garden (I'd be scared they try to get out).
Having said all that, if your dogs have lived happily in the house till now, I'd let them continue to do so. For me, my dogs are part of the (extended) family, and it would feel wrong to shut them outside. (It was different when we were temporarily part of someone else's 'family' on the farm, and we had to go along with their 'rules' then.) Mind you, I don't give my dogs free rein in my house either. They don't come upstairs ever, and overnight and if we are out they are shut in the (large) hall (with radio 4 on)!
Please don't take this the wrong way. I don't mean to be polemical. I don't think it's cruel to put dogs outside overnight. I just don't see why you'd choose to do so.
(V. envious of you living in Portugal. My toes are freezing sitting here now!)