Final answer:
The aunt decided to work in the garden instead of the other things she had to do that afternoon because she needed to pick berries to make jam for the children.
Step-by-step explanation:
The aunt decided to work in the garden instead of the other things she had to do that afternoon because she needed to pick berries to make jam for the children. This is mentioned in the passage where Nanny is going down to the store to buy some more embroidery and thread, indicating that she has other tasks at hand. However, the aunt's decision to pick berries for jam shows her commitment to taking care of the children's needs.