Who takes care of Sophia's children while she is in jail? Why is this ironic, yet very sweet?

You're right, Odessa takes care of the children. Sofia and Odessa are very different. Odessa is strict, Sofia is lax........ but through these two characters and their differences we see how strong a bond there is not only within families, but within female relationships. Ironically, when it comes to Sofia's children, Odessa takes on their mother's traits. Harpo describes Odessa in this way:

"They love her, say Harpo. She let ’em do anything they want."