A flash outside with auto mode does not sound like a way to get a lot of great pictures back. If it's an overcast day I would forget about the flash, not a lot of 1 year olds wear hats and that would be almost the only reason to use it on an overcast day. Since you asked about auto I would say shoot in P mode, I don't think the kids will be moving all that fast at 1, if they are the Tv mode Patrice mentioned would work great turn it until the number say 125-250. If it's a bright sunny day you could use a fill flash but I think you will ruin more photos than get good if you don't know how to adjust the flash for different distances, at a park some of the shots might have some distance to them so you're talking about an expensive flash to fill those distances. Personally I would just try to take as many pictures in the shade and the rest will just have harsh lighting, I'm assuming this is more for fun and not a paid shoot also.