If we take a young child weighting 20-30 kg, 5 kg would be 1/4th to 1/6th of body mass... 15-20 kg or 10-15kg carried daily by adult is not a light load. And children have less developed bodies.
Id be interested what Grade 1 girls are carrying around that’s totalling 5kg - that’s three hardcover undergrad physics textbooks lol. And even if they do combine to 5kg, there’s no evidence it causes any harm to children!