"Those children were in Delhi yesterday."
Translation:वे बच्चे कल दिल्ली में थे।
Well strictly speaking the English translation of both sentences would be the same. However, to give you a feel of how they are different:
If I ask "where were the children yesterday?" "वे बच्चे कल कहाँ थे?". The answer would be "वे बच्चे कल दिल्ली में थे|"
But if I were to ask "When were the children in Delhi?" "वे बच्चे दिल्ली में कब थे?". The answer now would be "वे बच्चे दिल्ली में कल थे|"
So, basically, the focus of the sentence changes. You use intonations at different words when saying those sentences.
Please note that in English the answer to both the questions would be the same.
I hope this helps a little.