We eat like this all the time. We eat less 'desert' and 'snack food's than other people.
Breakfast often has cooked eggs, dried meat (though most has some sugar derivative like glucose, dextrose, Maltese etc apart from Parma ham which is expensive!) rather than cereal and milk. We make rice flour pancakes with eggs and rice milk with dried meat, almond butter, homemade jam (bag of frozen berries, heat up and boiled a little with some chia seeds to thicken).
Mains are:
rice, chicken, spring onions, cumin and egg
Roast chicken and roast potatoes
Salmon, rice, roasted sweet potato cubes with rocket
Rice pasta (not corn) with cheese free pesto
We generally start a meal with bowls full of raw carrots, cucumber, fennel etc
We do eat fruit but not a huge amount due to the sugar issue.
For snacks we have rice cakes, raw vegetables, some fruit, date balls (with cashews or coconut and then either dried raspberries or cacao powder.)
They will be used to this diet so don't feel like you need to adapt your diet to fit theirs.
Ask their parents for suggestions!!
Don't add sugar substitutes or even lots of fruit where you would ordinarily have sugar. If they are sugar intolerant, they may also limit fructose.
I make banana bread with buckwheat flour, eggs and cinnamon.