Common colds and fevers are typically caused by viral infections, for which antibiotics like co-trimoxazole are not effective. Using them unnecessarily can cause side effects. If a bacterial infection is present, the dosage of co-trimoxazole must be carefully calculated by a pediatrician based on your child's specific weight and age. For a simple cold and fever, keeping your child hydrated, rested, and using age-appropriate paracetamol for fever relief is usually best. Please consult your pediatrician so they can examine your child and provide the correct, safe dosage if an antibiotic is truly needed.