I actually find self-catering much more relaxing. I don't mind being full catered for a long weekend say, but any longer and I'd rather self-cater.
I would echo the last poster's comment. Do you cook at home? What do you normally eat? Just try some variation on that.
For what it's worth, this is our rough eating plan on holiday:
breakfast - cereal or toast.
lunch - either take a packed lunch with us for a day out, or occasionally we grab a pasty/sandwich or whatever when out.
Evening meal - for a week's holiday we would eat out 2-3 times, one of those possibly being a fish and chips on the promenade sort of meal.
The rest we cook, but only things that are quick and easy. It's often a welcome break for the kids to relax inside after a day on the beach or sightseeing or whatever.
Pasta with a home-made tomato sauce takes about 30 mins. Or cheat with a bought sauce if you'd rather. Or do a spaghetti bolognaise, again quick and easy.
Baked potatoes with tuna mayonnaise and a side salad. Or whatever toppings you like for your baked potatoes.
Lots of salads - vary the protein content and the carbs, so you might have a simple cheese ploughmans or quiche or cold meat or whatever. Serve with potatoes or rice or just nice bread.
Things on toast - beans on toast with grated cheese on top is quick and yummy and can be a 'proper' meal. Ditto toasted sandwiches. Again, serve some salad on the side if you like.
Baked fish - takes about 30 mins, cook the veg while the fish is in the oven.
Sausages - sausage casserole, quick and easy, just bung in the oven. Sausage and mash with veg or beans, or sausage sandwiches served with crisps or chips or salad on the side.
These are all things the whole family should be able to eat - my children would eat them at the sort of ages yours are.
If you really don't want to do much cooking, just shop for a nice selection of different cold meats, cheeses, salads etc and serve them up in different combinations with different sorts of bread, potatoes, chips, pasta etc.
Try to have a few ideas in mind before you go out, just in case you get stuck for ideas.