I have found with the exercises in Lingua Latina (and a lot of other textbook composition exercises) that it is frequently possible to recycle a great deal of the vocabulary/grammar of the question and use that in your answer. So, a good answer to "What was the use of the shields?" might start with "The use of the shields was..."
It looks like you are looking for an answer that will state "The use of the shields was to carry the grandson."
You can recycle the phrase "The use of the shields was..." from the question, and you're left to supply "to carry the grandson." My suggestion is that you render this phrase with the accusative of nepos, nepotis, m. (because it is the what is being carried) and the infinitive of whatever verb you would like to use to translate "to carry" (perhaps portāre, ferre).