Just a thought, has she been tested for Type 1 Diabetes? One of the symptoms is drinking a lot of water. Also diabetics can have a sickly sweet smell at times, depending on their blood sugar or ketone levels. It might be worth asking the doctor, if only to rule it out
I was about to suggest this. Prior to my daughter being Diagnosed as T1D she was back in nappies 24/7 and soaking through them. She always "smelt" even if I changed her hourly. She also had a horrible pimple looking rash. (she was 2y 8m at diagnosis so a bit different in age)

Although now at 6 she can still wet the bed and only came out of night nappies Xmas 2010.

I do hope it's not that serious though

Feel free to PM me as well if you have questions