Let them get the measure of it.
I enjoy teaching measurements especially capacity: how much a container holds. It is a lot of fun when water and jugs are involved!
I do think it is the best way to give a child the idea of how much a litre/half a litre is or how much a pint/half a pint is.
It can be useful to be familiar with both the modern metric measurements(litres, millilitres etc.) and the imperial measurement system (pints/gallons etc.)
A suggestion:
Let your child/children help with cooking, allowing them to join in and measure out the ingredients,
as long as it is done safely, as this can be a very practical and helpful way to teach a child.
Get them to read the recipe with you and do each step together.
It might test your patience, but it gives them an every day application of what they are learning at school!