I use Banana Boat Kids SunBLOCK spf 30 or Hawaiian Tropic Kids Sunblock spf 45. I'm quite fond of Kokatat's spf shirt and my big-brimmed hat. I will burn even with sunscreen, so physical blocks (clothing, or sunblock with titanium dioxide or zinc oxide) are very important.
I've tried several varieties of "pure" titanium dioxide sunblocks, but I've always sweated them off, or they've washed off in water. I only use them if I'm not gonna sweat or get wet. They're much more expensive so it's frustrating.
I keep going back to my old favorites (banana boat and Hawaiian tropic).
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson