Grate the cheddar and Parmesan and measure the milk before turning on the hob. Once butter and flour meet, the sauce benefits from steady attention, so this preparation prevents a child having to rush between a hot pan and the grater.
Cook the macaroni according to its packet because shape and brand affect timing. Aim for tender pasta with a little bite: it will soften slightly more when folded through the hot sauce. The adult-led draining stage is a useful pause before everyone returns to the sauce.
Let the butter melt without browning, then stir in flour until no dry patches remain. Cook this paste for 1 to 2 minutes so the finished sauce does not taste floury. It should bubble softly and smell a little biscuity, not turn dark.
Treat the first milk additions as a series of tiny experiments. Each splash briefly makes the paste look thicker; whisking then turns it smooth again. Once the sauce loosens, children can pour more steadily, but somebody should keep the whisk reaching the pan's edges and base.
The white sauce is ready for cheese when it steams and leaves a clear coating on the back of a spoon. If it is watery, cook it for another minute; if it is very stiff, whisk in a splash of milk. Lower the heat before cheese goes in and never chase faster melting with a hard boil.
Fold in macaroni rather than beating it, then decide immediately whether the sauce needs a final splash of milk. It should look a little looser in the pan than on the plate because pasta continues drinking it in. Serve promptly and cool children's portions before eating.