Chicken traybake goes most smoothly when you set up the tray before children start chopping. Choose a large roasting tray, put it on a stable surface, and decide where raw chicken will wait separately until it is needed. That separation keeps the child's vegetable work calmer and makes the hygiene boundary much easier to hold.
Match the knife jobs to the child in front of you. Carrots are firm and can roll, onions are slippery once peeled, and peppers are easier once an adult has removed the stalk, seeds and ribs. It is completely fine to pre-chop one ingredient and let the child do another. The aim is supported independence, not proving they can manage every tool in one recipe.
The tray mixing is the heart of this recipe for children. Let them add potatoes, vegetables, garlic, flour, oil and herbs, then use clean hands or two spoons to coat and spread everything out. Talk about a single layer: food in a heap cooks unevenly, while food spread out has room to roast. That is a practical cooking idea they can actually see.
Keep raw chicken as a deliberate handover. Use tongs, place the thighs on top, then pause for handwashing and surface cleaning before anything else happens. This can feel like a grown-up interruption, so I would frame it as part of the recipe: raw chicken has its own rules, and good cooks follow them.
Boiling stock and oven work are the other adult-led stretches. Spoon or pour the stock around the tray slowly so it does not splash, then let the oven do its long work. If your child gets impatient, give them an observation job: notice the smell changing, check the timer, or predict which vegetable will soften first.
At the end, do not rush the serving just because dinner is ready. The tray is heavy, the juices are hot and the adult needs to check the chicken properly. Once it has rested for a few minutes, hand the recipe back where you can: children can spoon vegetables onto plates, choose a piece of chicken with help, and see that the tray they built has become a proper family meal.