That does seem like a healthy low fat diet for an adult or older child, bud little ones needs more fat in their diet. Not much more but a bit more! That's why they have whole milk not skimmed or semi skimmed. Fat helps you to feel full and satisfied.
For breakfast you could do the weetabix and banana, but also a full fat fromage frais, a slice of cheese, a slice of toast with butter or peanut butter.
Snack you could add some full fat Greek yogurt to the pear, or switch it out for some crudités and hummus
Lunch instead of the cheese triangle have a slice of cheese. Have scrambled egg with one whole egg and an extra yolk, or one egg but a little bit of butter. Use full fat butter not margarine on the bread.
Snack grapes are white sugary even though they are good for us, they also spike blood sugar (most fruits do) so try to have more veg than fruit. Another way to balance it out is to have some fat or protein with it. So have some grapes and some chicken cubes or avocado, or skip the grapes and have some crudités.
Dinner Instead of tomato and veg sauce, you could add some cream, or some cheese. You could have something with mince like Bolognese for protein.
The fish finger dinner I would do 2.5 or 3 fish fingers but less potatoes. A little bit of butter on the vegetables. Maybe a mayo of yoghurt based dipping sauce. Broccoli and carrots are great, but peas have a higher protein content so can be more filling.
Afters is the natural yogurt low fat? Natural yoghurt is not as filling, dense or protein rich as green yoghurt. Always chose full fat, full fat Greek yoghurt being the best for fat and protein content. You could also add some crushed nuts.
It's not always about increasing calories to keep them full, but looking at what the calories are used on. Obviously fruit and veg are marvellous, but they should not dominate a child's diet at the expense of fat and protein and wholegrains. There needs to be balance.