The arch is there, but until around 3 or so it's hidden by a pad of fat on the bottom of the foot. This is perfectly normal and necessary for the development of the foot. As their stride becomes more adult, and as they lose their baby fat, the arch will slowly emerge. Shoes with arch support undermine the natural development of the strength of the foot, and are not a good idea. In generations past, it was believed that it was necessary, but we know now that it is not. That is one reason why so many of us older folks have trouble with our feet and also with our backs and posture!