You very carefully don't state the sex of the younger child. If they're female then not only a growth spurt could be imminent but onset of puberty.
My Dd was like this to a point just before she reached that point, then when it hit she had a massive height spurt and then returned to her normal eating level which was actually a problem as it edged her into underweight and I had to work really hard to get her to consume enough cals.
Her weight is partly due to her disability too though, which was undx at this age.
She's prone to seeing food as a necessary evil rather than something to be enjoyed which is a bit odd as her dad and I both love our food (though he was a fussy bugger when I met him - whole other thread really but parental issues a factor)
If their weight is ok, they're active, then I would
Improve the meals and portions - a bowl of cereal and tuna sandwich are not sufficient through the day if they are as I suspect the British basic versions. Plus most cereals and milk are high in sugar and while I'm no "carbs are evil" poster it's not healthy to have carbs as the main nutrient in the first meal of the day.
Reduce the less healthy foods in the house and/or make them less easy to access, I don't mean in a pointed way, just in a make the healthier stuff easier to grab, more appealing etc
Encourage spreading calorie intake across the day not mostly in the afternoon/early evening
Are they drinking enough? A common occurrence as many dieters will tell you they've learned is mistaking thirst for hunger. Many of us don't drink as much as we should (I'm guilty of this myself over all my life! Even now I'm very aware of it I can still have days where I develop a dehydration headache and then - d'oh! - realise it's 6pm and I've had one small drink all day! Did it this weekend. Stupid but easily done)
Are they bored/stressed/anxious/depressed? Mood can make a huge impact on eating habits.
It's been a bloody tough year for many and food is one of the few pleasurable things we can enjoy without having to consider covid rules and logistics!
So give them a break and help them develop a healthy relationship with food.