To me it looks like Miku and Kaito took bits off each other's plate. Meiko purposfully made Kaito's horrible, thus the grim Miku at the end. The grim Kaito comes simply because Meiko beat the crap out of him for eating Miku's food.
Actually, I think it's the opposite. KAITO is giving his... whatever he has on his chopstick to Miku, exchanging it for Miku's vegetable. Since kids don't like them and whatnot. The moving lines seem on the third panel seem to support that, since she seems to be moving her arm towards Kaito's plate (to put the green thing on it) rather than from it.
After all, KAITO does seem like the doting type.
MEIKO smacking KAITO could be because 1) she thinks he's spoiling her, 2) she believes in a nutritional breakfast / lunch / dinner or 3) she thinks it's an insult to her food.
well, in essence it's the same as they are eating off each other's plate. And obviously Meiko can't stand for Kaito getting near Miku or sharing food with her.
