I'm a bit late coming in..DS2 just had a bout of croup. He took the predimix for 3days once a day. The cough was better within hours of taking the first predimix. I mixed the predimix in a little bit of DH's gatorade to take away the taste. The expiry on my bottle is next year and the pharmacist said it will not go off after opening and the expiry on the bottle is the actual expiry even after opening. I keep it in the fridge. I also have repeats just in case, as Trillian said, it's a creeper and one minute they have just a plain old cold then all of sudden the seal cough!
The GP also suggested that vapourisers and moist air don't work for croup and if the cough gets really bad to take them outside into the night air if you are not taking them to hospital. I am not sure on that though, as in the outside night air?