Adult cold and flu medicines often combine several ingredients, such as a fever reducer, a decongestant, and a cough suppressant, in doses meant for adults. These are not suitable for young children, and decongestants and cough suppressants in particular are generally not recommended for children under a certain age because the risks can outweigh any small benefit.
A major hidden danger is double dosing: if you give a combination cold remedy that already contains paracetamol and then add separate paracetamol, you can exceed the safe limit. Always read the full ingredient list and avoid giving two products with the same active ingredient.
For most childhood colds, simple measures work best: fluids, rest, saline nose drops for a blocked nose in babies, and a weight-appropriate fever or pain reliever only if needed for comfort. A pharmacist can recommend what is safe for your child's age and weight and warn you about ingredients to avoid.