The Prophet Muhammad (pbuh) was chosen to tell the people there was only one God. This happened when Muhammad was 40 years old, when an angel appeared and revealed the first verses of the Koran to him.
The class could focus on the fact that Muhammad (pbuh) could not read and that the first words from God were "Read in the name of your lord". The children could look at Urdu writing and try to copy some of it using black paint and paint brushes. Just as Muhammad memorised many verses of the Koran, ask the children to memorise a poem that they like and then recite it to the rest of the class.