There are incredible desserts in Spain like flan and natillas (both are made with milk, egg, sugar and cinamon), arroz con leche (rice, milk, sugar and cinamon), tarta de Santiago (almonds cake),...
Nobody has mentioned pasta, which makes a major part of our regular eatings nonetheless, specially maccaroni with tomato and chorizo. Chorizo? Every kid likes it, along with salchichón, sausages, cheese from Burgos (very soft), and serrano cured ham. I remember wonderful bocadillos of them...
Did I mentioned bocadillos? Yes, two halves of french bread cut along and filled with that stuff, or others, like tuna with red peppers, spanish omelette, lomo adobado, or even chocolate!
It will be a crime not to taste an orange juice
As for rices, you may try a sure champion: arroz con tomate (rice with tomato, olive oil and garlic) and arroz abanda (like paella without fish and seafood pieces in it). Which leads us to seafood and fishes, try gambas al ajillo (small prawns with garlic), lenguado, merluza or rape (shave). And also patatas alioli (potatoes with mahonese with garlic) or patatas bravas (potatoes with a spicy tomato sauce).
And meat: cordero (roasted lamb), steak, roasted chicken,...
There are an infinity of things to taste in Spain, of which kids like many of them. You should try to drive them wisely to taste those things.
As you know them, you can try things first and you will instantly know what they'll think of them.
Good luck and let us know your experience!
Fernando
PD: I can give you the recipe of any of the dishes above