I think that Sojourner Truth used the word children because it is how she would explain things to her children. This word signifies that the following words she will be uttering are of dire importance. She connects to the audience by making them remember that they were once children and they had to listen to the wisdom of their elders, especially, their mothers.
Sojourner's use of the word children emphasizes her being a woman and a mother. Someone who is worthy of being listened too as she imparts words of wisdom.
