I know I'm late in commenting on this, but I remember all too well what it's like to be forced to stay inside and do schoolwork when all you really want to do is go outside and play with your friends (although I grew up in Nevada, so at this point in the year we were generally trying to stay inside, as it was too hot to be outside). However, even though it frustrates you and your children at times, ultimately, you're going to be happy you stuck with it and they're going to be grateful you cared enough about their education to have done it. I'm so happy I was homeschooled and I plan on doing it for my own children someday (granted, when I think about the battles my parents had trying to teach me math, I shudder, but it was worth it).

Keep at it, and maybe (in the future) on a really nice day when nothing productive is being done anyway, feel free to call a day off. Most homeschoolers are so far above their age group in terms of grade level that it doesn't hurt, and if they're so focused on getting stuff done so they can go play they're not actually learning anything, it's not worth the battle.