how can I help my child see better?

Depends on age. A baby or young child who seems to have a seeing problem should to see a pediatric ophthalmologist (eye doctor for children) to diagnose the problem. Sometimes, no problem is found. Other times, glasses are needed. Less commonly, there is a more serious issue in the eyes. Older kids can see a general ophthalmologist and be checked for nearsightedness, farsightedness, astigmatism, etc...
Why? If they are having problems with visual acuity, they should be seen by a pediatric ophthalmologist. If you mean perception, then they should be seen by a neurodevelopmental pediatrician.