I tell mine first to drink a glass of water - often thirst is mistaken for hunger. If still hungry then the go-to is peanut butter & jam sandwiches on granary/seeded bread - mine learned at age 8 to make their own with sliced bread and a butter knife without making a big mess in the first lockdown when DH and I were trapped on zoom calls. Apple, satsuma or a banana on the side.
Big plate of sliced veg and hummus, peppers, cukes, carrots, raw cauli, raw broccoli, raw mushrooms. I don't see the last three offered raw in the UK so often but they are a standard crunchy raw veg in the USA.
I also still do a once a week seeded muffins (often a "morning glory" style with grated carrots, pumpkin and sunflower seeds) or a homemade loaf bake of some kind for the mid-afternoon dip - usually banana but I have also done yoghurt blueberry, marmalade, seed cake, and homemade malt loaf with bran flakes (my MIL's recipe). To each I add extra nuts, seeds, wheat germ etc to make a bit more than just cake.
If the kids say they don't fancy a piece of cake then they must just be bored, not hungry!!
DS1 at age 12 is increasingly making instant ramen as a snack but I showed him videos of how Japanese people add ham, boiled egg and vegetables to theirs and now he adds all those to make it more than just white carbs in an MSG broth!