I run out of cooking idea and look for good cook book for child. Which one should I buy?

Perhaps you can try one of Jamie Oliver's cookbooks. Being a father and a proponent of healthy cooking as well as good nutrition for children, you can get plenty of great recipes from his cookbooks. You can try out some of his recipes here: http://www.jamieoliver.com/family-food/ - they are categorised into helpful sections like "Weaning food", "Family Basics" etc. He also has a YouTube channel called Jamie Oliver's FoodTube: https://www.youtube.com/user/JamieOliver