It neither shortens the length of an illness nor prevents complications. The only medical conditions for which bed rest is helpful are heart failure and respiratory failure. Children with these serious problems choose to remain in bed because they feel especially bad. When children are sick, it is all right for them to choose their own levels of activity. If they are feeling really ill, they will want to be in bed.
Children naturally decrease their level of activity when they are sick.
We probably don't trust their survival instinct because we know too many adults who go full speed ahead when they are sick. Forced bed rest is impossible to achieve. If you insist that your child stay in bed and then you leave the room, when you return you may find him using the bed as a trampoline.