Final answer:
The nurse should place the child in the lateral decubitus position, lying on one side with the back curved and the knees pulled up towards the chest.
Step-by-step explanation:
The nurse should place the child in the lateral decubitus position for a lumbar puncture. This position involves lying on one side with the back curved and the knees pulled up towards the chest. It allows for easier access to the lower lumbar area of the vertebral column, where the procedure is performed.