Wash the basil, then dry it thoroughly between clean tea towels or in a salad spinner. Water clinging to the leaves dilutes flavour and can make the pesto look thin before pasta water is even added. Grate the Parmesan and measure everything before the saucepan boils.
Choose a processor bowl small enough for this modest batch. Put basil in first, then garlic, pine nuts and Parmesan, with oil over the top. If ingredients sit above the blades, pulse, switch off and unplug, then let an adult scrape them down rather than adding water immediately.
Treat pasta water as an ingredient. Place a heatproof mug beside the hob before cooking so the adult remembers to scoop it out before draining. Its starch is what makes pesto creamy around pasta even though the card includes no cream.
Blend with short pulses and judge the texture after scraping once. The pesto does not need to be perfectly uniform; tiny flecks of basil and pine nut give it character. Add water by tablespoons only when the mixture will not circulate or looks too stiff to coat pasta.
Use medium-low rather than fierce heat for the final toss. Add pesto, pasta and one small splash of water, then fold until glossy. If it pools at the bottom, keep tossing for a minute; if it looks dry, add one more spoonful of water.
Serve as soon as the coating looks right because pasta keeps absorbing moisture. Extra Parmesan and basil are optional finishing jobs children can control. Cool any leftovers promptly rather than leaving them in the warm pan.