There are days when a child in the thick of treatment keeps pushing their food away. Even adults don't want to pick up a spoon when their appetite is gone, so imagine a child on chemo, whose mouth is sore, who's hypersensitive to smells, and who feels queasy on top of it all. The first trap most parents fall into is "just one more bite." The intention is understandable, but forcing spoonfuls in only makes the child come to dread the dinner table itself. It's better to cut the amount down and make each bite count more.
When there's mouth sores (stomatitis) and everything is raw and painful, irritation is the real enemy. Set aside anything hot, salty, sour, or rough for now. Lukewarm porridge, mashed potato, soft steamed egg, or pudding-like textures that slide right down are much easier to manage. Keep the seasoning blander than usual. Highly acidic things like tomato or orange sting the sore spots, so it's wise to leave them out depending on how the day is going. Drinking through a straw lets food get past the painful areas with less contact, so a small trick like serving thin rice gruel or a watery smoothie through a straw can work surprisingly well.
There also comes a stretch when a child becomes unusually sensitive to smells. If the steam rising off warm food is enough to trigger a gag reflex, try deliberately serving food lukewarm or chilled. Cold yogurt, chilled fruit puree, or cooled-down potato pancakes tend to go down more easily. When you cook, keep the child well away from the kitchen and air the place out thoroughly. It helps to think of it as something like morning sickness.
Even so, the days when they just won't eat will come. On those days, don't push to fill up the meal; instead, shift your aim toward boosting calories and protein in a small amount. Stir an egg yolk, a spoonful of perilla oil, a little cheese, or finely ground tofu into a bowl of porridge, and the same portion becomes far more filling. For drinks, give a banana blended into milk, or a nutritional supplement drink with protein in it, in small amounts and often. Let go of the rigid three-meals-a-day frame; a bite or two every two or three hours is more realistic in this period.
When it comes to hygiene, it's reassuring to be a notch stricter than usual. Since immunity is down, set aside raw food, undercooked meat or eggs, and unwashed raw vegetables for now, and serve everything thoroughly cooked and warm. Use water that has been boiled and then cooled, and don't leave leftovers sitting out for long. Remembering that a single bite that actually makes it into the child today is worth more than some elaborate "restorative" dish can also lighten the guilt a little on the days you couldn't get them to eat.
What's written here is only meant to help with care at home. The foods that are okay to give change depending on the child's condition and the stage of treatment, so please be sure to decide together with your attending medical team or a dietitian.