As someone who manages a childhood obesity project for a living, could I suggest something healthy but fun to make.
We make vegetable soup and soda bread with the kids which they love doing, there's lots of chopping to do and they love the effect of mixing lemon juice with milk to make a wierd yogurty substitute for buttermilk in the bread - what's great is that the bread can be made in a short enough time while the soup cooks and their attention span hasn't wandered. Just use up any veggies, tinned tomatoes, some water and a veggie stock cube - needs about 35-45 minutes simmering time.
The soda bread uses three cups self raising flour, 2/3 cup milk to which a teaspoon of lemon juice has been added, a pinch of salt. Mix together into a squidgy mess and shape on a well floured board. Cut a cross in the top and place on a baking tray which has been lightly greased. Cook for the same time as the soup in an oven at 200 degrees C.
Another great idea is a home made pizza using fresh veggies cut up to make face, i.e. sweetcorn teeth, red pepper mouth, mushroom eyes, etc - it's cheap, great fun and you can use soda bread dough as the pizza base.