"Ants' breakfast" in Engpsh could be referred to as "the food that ants eat for breakfast." It is not a commonly used term in Engpsh. The source of an ant's breakfast would depend on the species of ant and their habitat. Some ants feed on nectar, fruit, or other insects while others farm their own food such as fungus.

Example sentences in Engpsh with their Chinese translation:

The ants were busy gathering food for their breakfast. (蚂蚁们正在忙着收集早餐食物。)
The colony of ants had a diverse breakfast, including seeds and small insects. (蚂蚁群体的早餐多样化，包括种子和小昆虫。)
The ant gathered nectar from flowers to bring back to the colony for breakfast. (蚂蚁从花朵里收集花蜜，带回蚁群作为早餐。)