why no hats or sunglasses? if they are fair, they likely have blue eyes which puts them at much greater risk for cataracts when they're older - one more thing for you to worry about! :)
if you're using sunscreen and are good about reapplying, relax.....I find that I start to burn before they do so I simply monitor myself rather than worrying about them.
mine are starting to fight the swim shirts (they're almost 9 and 11) because it seems no one else uses them...argh! but in the end they hate sunscreen more than they are embarrassed, so they still wear them. my husband and I also wear swim shirts, so that helps with getting them to do it.
we do try to avoid the beach/pool between 11 and 2. I don't worry about amusement parks so much because you're in and out of the shade - ditto on parks.
I don't put sunscreen on them during the school year so by the time school is out they have a slight tan from recess - that helps. I also no longer put sunscreen on them for running around the neighborhood - again gives them a slight tan which helps for the burning (I know, I know doesn't help on the UVA and cancer....) . I figure it also gives them their vitamin D.
I also use really good sunscreen - physical blocks only - no chemical. I'm far more concerned about the chemical reaction in chemical sunscreens (many of which may carcinogens) than I am about a slight burn. If we know we're going to be outside with little or no shade for the entire day - I insist on lightweight long sleeve shirts - problem solved!
You need to stress to the inlaws how easy it is for them to burn. and if they are over the age of 6 - they should be carrying sunscreen/sunblock with them and using it - mine have for many years now. My IL are not that dark-skinned but flat out don't believe in sunscreen/sunblock. My kids simply say they need it. Mostly because each has gotten slightly pink once or twice and they absolutely hated sleeping covered in aloe! Made them very good about using sunblock. Or simply dress them in lightweight long sleeves and hats when they go with grandparents.
if she didn't get freckles to age 4 or older - relax - you're doing fine. even if younger, if she hasn't had a burn, you're doing a fine job.