As the parent of a child, I have downloaded a number of different apps created for children of varying ages. I have found that quality can vary quite a bit, but I've seen some great apps that would be considered truly educational and easy for a child to use. I think these apps are a wonderful invention.
Children can learn from baby apps but they really should not be used. Although a baby can learn from various apps, it stimulates the brain in a way that a young brain should not be stimulated. It is no different than putting your child in front of a television. That can be educational too. It can also lead to ADD, ADHD, learning problems, and social issues.