You may like to package your younger child's lunch in a special lunch box with separate compartments. The insulated boxes, such as the one shown here, are ideal to keep cold foods fresh until lunchtime or hot foods hot.
Older children may prefer to tuck individual items into spare pockets in their school bag. You can use plastic bags and small containers to prevent spills and leakage here.
Tiny lidded containers are also useful for salad dressings.
Some foods, such as yoghurts may require a spoon. There is a likelyhood that this could accidentally go astray so you may prefer to stick to plastic cutlery or, to be kinder to the planet, why not purchase a job lot of cheap teaspoons from a charity or Goodwill shop.
Greaseproof or waxed paper, along with tin foil can be useful for wrapping foods such as open sandwiches, so they don't fall apart in the lunch box.
To help with cleanup after lunch you might also like to pop in some wet wipes.