This is a very broad question. Therefore, it will be impossible to go into much detail within the usual word limit. However, I will give you some ideas about what you might want to cover in an essay about this topic.
First of all, there are two types of development you might want to look into: the cognitive development and the physical development of a child.
Firstly, I would like to point out some milestones with regard to cognitive development, which you might want to cover when discussing this topic further. In the first three months of their life, babies develop the ability to distinguish between different colors and tastes. From around six months, babies are able to recognize familiar faces and sounds. For example, a baby will be able to recognize their parents' faces and voices at this stage. When babies reach their first year, they will be able to produce sounds and they will have started to communicate by imitating sounds and facial expressions. A very significant cognitive milestone is reached once a child is able to tell the difference between "me" and "you," which usually happens between one and two years of age. Other important cognitive milestones in the life of a small child up to preschool age include the ability to sort objects according to size, identifying pictures of objects and the development of an increasingly growing attention span.
In terms of physical development, there are far too many milestones to mention. However, you definitely should explain in your work that the most important parts of physical development take place in the first two years of a child's life.
In the first six month of its life, babies will learn to lift their head and to turn from their back onto their tummy. Once a child turns one year old, the child should have learned to sit unaided, to crawl, and to stand whilst holding on to something. The baby will also be able to pick up small items of food in order to eat. In the second year of its life, the baby will learn to stand and to walk unaided. Other important aspects of physical development, which develop until a child reaches preschool age, are the ability to run and jump, for example.