The frontal and maxillary sinuses
are air filled structure around the air way , and they are small in size in children and grows with age,
In 2 yrs old child hypoplastic maxillary and frontal sinuses is normal.
is protective mechanism as a part of body immune / defense system against infection, and the reaction is more in children as this system is learning pattern of infections.
Adenoids are located along the airway tract in posterior part of nose.
When they adenoids over react the adenoid size increases and cause obstruction to air intake via nose
Child opens mouth to take breath.
Same is a cause for difficulty in sleep.
As for as treatment part is concerned you need to consult your doctor.