Thank you for your enquiry. The most common cause of stuffy nose in toddlers (age 12-36 months) seen in my practice is "sensitive nose" (medically known as rhinitis). Rhinitis can be allergic (triggered by a protein known as an allergen, for e.g. dust mites, pollens, cat and dog fur, etc) or non-allergic (triggered by a change in temperature, cigarette smoke, strong perfumes etc). Common symptoms of Rhinitis include nasal blockage, runny nose, sneezing, itchy and watery eyes, postnasal drip (mucous dripping down the back of the throat, resulting in clear phlegm and cough).
To reduce the symptoms of rhinitis, avoiding the known trigger (if possible) is important. As dust mite allergy is very common, it is a good practice to minimise exposure to soft toys and to wash your bedsheets, pillowcases, and blanket covers at least once a week with hot water (at least 60 degrees Celsius high to get rid of the dust mites). For toddlers older than 2 years old, the use of nasal steroid sprays (to reduce inflammation) and antihistamine (e.g. zyrtec or aerius) may help. Saline sprays are also very safe and effective to unblock dried or thick mucous.
You should bring your toddler to a general practitioner or ENT specialist if the stuffy nose is persistent or causing discomfort.
Hope this helps and all the best!