We have been camping with our daughter since she was 8 months old. We bought some blow up tubes which slot into a bed sheet (they are called Dream Tubes and are designed to go in a kids bed to stop them falling out) and we put them across the downstairs bed width ways by the boot and we sleep upstairs. We have also left the rear seats up and our daughter has slept on the mattress in the boot (sounds a bit unkind, but she loved it and it was very safe...). The travel cot we've got doesn't quite fit in the floor space and is too tall to go on the bed. Our daughter is now 2 and we'll be trying her upstairs with the child net and Dream Tubes in the summer (which I hope means we'll be able to go to the toilet after she's gone to bed without waking her up...!)
I hope this helps.
Steve