I would say don’t be afraid to deviate, you’re the boss, if you feel like remembering your childhood, do it. There is no rule here.

But each of the little stories of your journey to Sudan should have the narrative engine of all stories — a character (you) who wants something, and is met with some sort of obstacle, and in the end some resolution of this conflict takes place. There are very few interesting anecdotes that don’t feature these elements, so keep that in mind. Even though you are writing the true story of your time there, the reader wants to be able to root for you, just as she would root for a character in a novel.

A theme is good but it should be a theme that emerges from a narrative goal or predicament, not one that is just presented as a “thought”.

Hope that helps.

(I consider Moveable Feast by Hemmingway to be a good memoir — it’s not just about Hemmingway, it’s about Fitzgerald and others. So the supporting characters are just as important as you, the main character).

(And Hemmingways advice to writers might be appropriate: “Think about what moved you about the experience you had, and write it so the reader will be moved in the same way.”)