Prepare onion, garlic, tomato purée, herbs, opened tomatoes and grated Parmesan before the mince comes out. Put a clean plate beside the mixing bowl. This keeps raw-meat contact contained and means nobody opens a drawer with sticky hands.
Add beef, egg and breadcrumbs to the bowl and mix only until the breadcrumb disappears. Divide into quarters, then make four balls from each. Children do not need perfect spheres; gentle, equal portions stay tender and cook more predictably.
After rolling, stop for a deliberate hygiene reset. Wash hands with soap and clean the bowl area, tap and every tool that touched raw beef or egg. Move the raw-meat plate beside the pan and keep serving utensils in a separate clean place.
Begin with a cold non-stick pan and medium heat. Let the first side form a brown crust before turning, then use tongs or two spoons so soft meatballs are lifted rather than pushed. Add onion and garlic only when the balls hold their shape.
Simmer gently after adding tomato ingredients. A fierce boil can break meatballs and reduce sauce before their centres cook. Turn once, then cut into a large meatball after 10 to 12 minutes; extend the simmer with a splash of water if any pink remains.
Cook spaghetti to its packet time and let an adult drain it. Fold strands into sauce in sections rather than dumping one tight bundle over the meatballs. Serve promptly with Parmesan and check meatball centres in children's bowls before eating.