I know you said she doesn't like porridge, but it's such a good breakfast to avoid hunger. Do you know what is it about porridge that she doesn't like - Texture? Flavour? Heat?
I'm guessing that if she also doesn't like cereal it's the texture?
Couple of options that could deal with that: either wizz your oats in a food processor so the porridge is then much smoother when you make it.
Make oat bars packed with nuts/fruit/seeds instead (use honey rather than golden syrup) and have with a drink of milk. You can make big batches, freeze them and reach a couple out to defrost overnight.
If it's flavour, then that's an even easier fix as you can add pretty much anything to porridge to change the flavour.
If porridge is still a no-go then what about beans in toast for breakfast?
I agree with the above posters that you need more carbs. This is a typical school days food for my daughter's (just turned 9 and 6). Both very sporty and active, rarely complain of hunger unless it's about 10 mins before dinner or in the case of the eldest when she's actually thirsty!
Breakfast: bowl porridge made with either full fat cows milk or oat milk. Added into porridge: prune purée, frozen tropical fruit, nuts, a banana. Sometimes they have peanut butter and banana instead.
Eldest takes fruit or seeded crackers as a snack (alternates days); youngest sometimes has free school fruit, but not often.
Lunch: Cucumber or tomatoes, portion of fruit, sandwich on homemade poppyseed bread - fillings alternate between: tahini, red pepper houmous, cheese, marmite, very occasionally meat leftovers from a roast.
There isn't often time for a snack before activities, but when they do have one it's either carrot/celery sticks plus dip (houmous, tahini, peanut butter, cream cheese) or a carb (crumpets, toast, English muffins, wrap, bagel, Staffordshire oatcake, crackers) with a spread or cheese. My eldest has also started getting into having a bowl of unsalted nuts - pistachios and cashews are her favourite. Snack is accompanied by a glass of milk.
Out and about snacks included mini cheddars, veggie straws, seeded oatcakes but they don't have any of them very often.
Dinner is very varied but always includes a protein, carb and lots of veg. Dessert is normally plain Greek yoghurt, but sometimes it's rice pudding or unsweetened custard (made with milk and Birds custard powder). After dinner they often have a 'treat' : something from their chocolate box, biscuit, small cake.
Hope that helps 😊