As is often the case, it is much easier to identify problems than to figure out how to solve them. There are three main types of factors that cause children to drop out of school in developing countries, but good solutions are not easy to come by.
The first factor is cultural. In many developing countries, there may not be a culture that emphasizes the importance of formal education. Since parents and others have not had much formal education, people will often think that being educated is not something that is normal or typical. They may not value education and may therefore drop out.
The second factor is more economic. In many developing countries, there are many families who need their children to work to help support the family. This makes it very hard for them to keep their children in school.
The final factor has to do with the quality of schools. There are many schools in developing countries that are substandard. Teachers may not be well-trained. There may not be enough books and supplies (or there may not be any). Students may be undisciplined. All of these factors make learning very difficult and make dropping out seem more logical.
All of these problems are hard to solve. The second two could be solved by the injection of a lot of money into the system. The money could be used to train teachers, buy supplies, and maybe even pay parents for keeping their kids in school. This is relatively straightforward, but it is very expensive and is easier said than done. Changing the culture of a country is even more difficult as it can only happen over time and cannot be imposed by the government. The solutions to these problems, then, would take a great deal of money and time.