"They wake up at noon."
Translation:Elles se réveillent à midi.
As far as I know, for reflexive verbs, you always use "se" regardless of the number of subjects. For example, it's both "elle se reveille" and "elles se reveillent." This is a guess, but it might be in order to distinguish between the 3rd person possessive adjective, but, again, that's a guess on my part. I hope this helps!