I have sets of bunk beds but my child Sim always chooses the double bed over the bunk bed for a bed time story. The option are found in with the chat options, though your child can "ask <parent> for a bedtime story" or your parent can "read <child> a bedtime story". Both times the kid ended up in the double bed. Personally its what happens next that is really cute... the parent reads, kid gets drowzy and falls asleep, parent tucks the child in and kisses him/her good night. One of the cuter actions (still think child playing peek-a-boo with toddler is the best) the Sims have.