Final answer:
The nurse would instruct the mother to apply a cold compress to the injection site.
Step-by-step explanation:
The nurse would provide the instruction to apply a cold compress to the injection site to the mother. Pain and redness at the injection site are common side effects of immunizations, particularly with the DTP vaccine. Applying a cold compress can help reduce the pain and inflammation. It is important to note that these side effects are usually mild and resolve on their own within a few days.