Ok so my son has had an awful cough since before New Year's.(also 21 months) He finally woke up in such a bad coughing fit that we took him to the ER. They couldn't do anything for him there, since the fit had mostly ended but I put my foot down and went on the attack. I had been doing all of these things but not all at once:
#1 Saline. They hate it. They scream. But, it thins out those thick nighttime secretions, so hold him down and get both nostrils.
#2 Little coughs. this will actually cause coughing at first so I do it after the saline to help the loose stuff move around after the saline.
#3 Benadryl. Half a tablet. It will dry him out for the night--don't use it during the day, though, as it will thicken the secretions too.
#4 Cool Mist Humidifier
#5 Elevate the head of the bed or prop him up on a pillow
#6 Mattress protector and pillow protector to prevent allergens from exacerbating the issue. Wash sheets and comforter in HOT water.
#7 Lots and lots of water--like double usual. Water instead of milk at night. I swear it makes his nose worse, new claims are that it doesn't affect mucus secretions. I'm sure it does based on first-hand experience.
#8 Warm apple juice sipped through a straw. I use a coffee straw. Will settle a scratchy or dry throat.
(Also, Delsym, in my experience does.not.work.)
Good luck, my son's cough has really calmed down and he hasn't woken up coughing since Sunday night.