Hi welcome - this is a gastro-intestinal disease due to malfunction or absence of nerve cells stool does not come out properly.
So child need soft & bland diet. Very less oil , no spicy ,
sugar within 20 g,fat or no cream products ,no out of home foods.
Restrictions of milk like high fat contains rather yogurts , light cheese, etc good.need fight fibers in diet, avoid all types of fast or junk foods, ..
Requirement if consuming more water, fluids, low spices based but high protein foods.
Just try to add Porridge, upma, soft rice with mix veges, ,with egg white, tofu , Almond, added some time Apple, berries , enough dairy also good but avoid milk neither high fat or create lactose intolerance. Avoid all foods showing gas, acidity, intolerance.
It's better to visit a Dietitian personal to make a customers diet plan for your child.