Can I get a patent on a product that has been in use until now for adults and i want to start producing it for kids?

If the product is unchanged, you cannot obtain a patent on the product simply based on your proposed new use.  However, if you can convince the patent office that it would not have been obvious to use this adult product with children, e.g.,because children would be using it in a vert different manner unrelated to its use by adults you might be able to obtain a patent on the new method of use of the product..