I don't think my family is quite as fair-skinned as yours, but my dh cannot tan at all, his skin is either white or red. The only sunscreen he's found that can protect him for more than an hour or so is Coppertone Continuous Spray. I read about it here on the DIS! It sprays on for even coverage, and can be sprayed on the scalp without making your hair all greasy. It dries almost instantly. It's so easy to apply, that you're more likely to use it more often, and as others said, that's the important thing. It goes fast, though. Our family of 4 went through 2 full bottles in a week at WDW last fall.
As for the 13 month old, in addition to sunscreen, hats, etc, try to keep her in the shade as much as possible. If your stroller doesn't have a good sunshade, get one you can add to it. Make sure you have some light blankets to drape over her if she falls asleep in the stroller. Sometimes, staying shaded doesn't have to affect what you're doing, it's just a matter of being aware. Like someone said, cross the street if the other side is shadier. Find an indoor or shady spot to sit down for a rest. Even standing in a sunny line, position yourself so your body shades the kids a little.
Good luck and have fun!