If the superficial veins of the upper limb are visible and/or palpable, for an expected duration of 24-48 hours (for example, for a radiological exam, for a surgical procedure, for a bolus infusion of a one-time drug) the obvious choice is a short cannula.
In the child, 24G - 22G cannulas should be used. 20G cannula are preferably used only in case of expected infusion of contrast medium for radiological exams or in case of expected blood transfusion or infusion of blood products.