Praise be to Allah.The scholars have expressed different opinions. The moderate and most just opinion is that you return the entire original sum to the orphans plus half of the profits, as if you and they had been partners with their capital and your labor, hence the division.
This ruling has come from Imam AHED. It is also the opinion of Ibn Taymiyya which his pupil Imam Ibn ul-Qayyim prefers. This also applies to the situation in which you have rustled a cattle which gives birth to calves. The cattle must be returned and the calves must be split. If the original cattle dies then its price plus half of its calves must be returned.