Suggest Remedy For Bloated Stomach, Thin Body, Difficulty In Passing Stools With More Carb Intake In A Child

My 2 yr 8 month old daughter often has a big ballooned tummy, especially after eating, but fairly thin legs, arms and chest (you can feel ribs) she often struggles to do a poo. She has a good diet, home cooked meals, at least one or two types of fruit and veg a day and mostly water or milk to drink. However, she eats lots of carbs - cereal, bread, potatoes, pasta. How can I help her be more regular and stop tummy aches from needing to poo?