Must a wife who admitted to theft both before and after marriage be divorced, despite her oath of repentance, or should she be kept for the sake of the young child?

The best marital choices are those with religiosity and good character. Her engaging in theft and then repenting is not a sufficient reason to separate from her. Rather, you should be an aid to her in her repentance and steadfastness. Allah Almighty accepts the repentant among His servants and replaces their evil deeds with good ones. No human being is free from shortcomings, and the best of people are those whose flaws can be overlooked.