You can buy saline solution from your chemist, and put a drop of the solution into each nostril. They will more than likely snort or sneeze to get rid of the solution in their nostrils, and this will clear the nasal passages of any mucous build-up. If he or she doesn’t do this, then you can do it for them, by carefully suctioning the nostrils with a rubber bulb syringe.
If your toddler is two years old or thereabouts, there are a few ways that you can try to teach them to blow their own nose. In the meantime though, your toddler might want to blow their nose by imitating you, so let them, if they’re interested in doing it on her own, then they’ll learn to do it a lot faster.
Just remember to teach your toddler to blow gently, as using force to blow their nose could force any mucous to back up into their middle ear or sinuses, which could lead to infection.