Cold and chest congestion in a one-year-old baby are usually caused by a viral infection and often improve with supportive care. Levocetirizine syrup can be used in children, but the dose and age suitability depend on the specific formulation and the child's age. Medicines should not be given to infants without proper medical guidance. Consult a paediatrician to examine the baby and advise on the correct treatment and dose.