Can I put my 3-year-old son, who is congested, in his room with eucalyptus oil and a diffuser?

Yes, you can use a eucalyptus oil and a diffuser in your child's room, but it may not be very effective for clearing congestion. It is suggested to try other methods such as using Sinus Rinse™ or Ste-Justine's nasal hygiene technique for better results.