It probably is just his age.
Wheras years ago, the need to evacuate would wake him up, and he would ask to go out, in his old age, those signals just stop working, and his bowels will operate themselves while he sleeps.
If he has a bed, then you should look at getting some old comfy sheets or blankets that you can wash easily - there really is unlikely to be a solution that will stop him doing this.
You should make sure that your daughter is aware that these are the signs that your dog is getting old, and he's not going to live for ever. A little gentle preparation will hopefully prepare her for that morning when he has slipped away in the night.