In practice, whether you round the volume to be administered to 0.1 mL or 0.01 mL is going to depend on the graduation marks of the syringe that you have available, and not whether the patient is an infant or pediatric patient. During your course work your instructor should tell you whether you should round to the nearest 0.1 mL or 0.01 mL.
In my current practice I have the following syringes available for the administration of enteral medications: 1 mL, graduated in 0.01 mL increments; 3 mL, graduated in 0.1 mL increments; 6 mL and 12 mL, graduated in 0.2 mL increments; and 20 mL and 60 mL, graduated in 1 mL increments. When I calculate the volume of medication to administer I complete all calculations, and then round my final volume based upon the syringe I am going to use.
When using the 1 mL syringe, you can accurately dispense a volume to the nearest 0.01 mL. When using the 3 mL syringe, you can accurately dispense a volume to the nearest 0.01 mL, and closely estimate a volume to the closest 0.05 mL. When using the 6 mL or 12 mL syringe, you can accurately dispense a volume to the nearest 0.2 mL, and closely estimate a volume to the closest 0.1 mL. And when using the 20 mL or 60 mL syringe, you can accurately dispense a volume to the nearest 1 mL, and closely estimate a volume to the closest 0.5 mL.
In the example you provided:
1. How many kg does the patient weigh?
2. How many mg acetaminophen will you administer?
3. What is the calculated volume to be administered?
4. What is the actual volume to be administered?
5. Which syringe would you use to do so?