As a Paediatric SHO (in a different country) who still occasionally volunteers for the local ambulance service, my answer is load and go. IV cannulation in anyone is easier in the controlled and well lit emergency department. Plus, there'll be more than two people, so you can simultaneously have more than 1 person attempting venous access (who is likely to be more used to sticking lines in kids than you are) as well as someone else doing the initial assessment. Not a criticism of your skills, but just a reflection of reality. Diesel, every time.