Just do it at the earliest age possible, say 1 or 2 years old, before they even twig that they have a choice or can throw a strop. The sooner it becomes "just the norm", the less hassle you'll have in later years. We put sun cream on our son right from the first times we'd take him out in a pram without a sub umbrella over it - as a result, we've never once had a "refusal" - putting sun cream on became as normal as putting on his pants and T shirt!
But yes, as another poster said, choose the "right" suncream - we found a brand that was very milky which was easy to rub in, wasn't sticky, etc., and bought it in a spray, so it was a quick spray of the arm, quick rub in, then the other arm, then legs, then face - all done and dusted very quickly. The longer it takes and the messier it is, the more likely they're start refusing simply due to discomfort.
I was the same with OH - he used to make a song and dance about suncream - he preferred to go without. turn red as a lobster and moan for hours after! Until I found the milky spray on brand, and then he'd do it without me even asking him (or threatening to withhold privileges until he did!).