Don't worry you will manage ok with your budget which is about £66 a day for 15 days, after all the 11 year old won't be drinking alcohol which uses up more of your money. There are a number of cheaper places to eat, Fener cafe, Merkes cafe, Chillies, Oz Adana, Cafe vita (BBQ on wednesday nights (25 lira- £10 for 4 courses but you need to book, they have a website), Ali Babas, Photo Pizza, the new kebab place alongside Corner Restauant and others you will find by looking at the menu displayed outside the restaurant.
If you do things like boat trips you have a very good meal provided so you can manages with a smaller meal in the evening.
You'll have a great time.