A few options:
1) Nurse places IV (depends on who the nurse is)
2) Nurse holds mask while you place IV
3) Surgeon places IV (yes, to my surprise we actually have a couple peds ENT's that get off on starting the IV - good at it too)
4) My favorite (Thanks
@Noyac for the idea): place kid's hand over the mask sorta like he's holding it - this allows you to hold the mask down with your middle finger while while your index finger holds the wrist and your thumb holds the fingers down over the mask keeping the skin taut. Now pop in the IV while masking the whole time. Straight

status.