I don't kill them and we regularly get rattlesnakes in our backyard - and I have two kids.
What good is killing one or two snakes going to do? I bag them and deliver them to a wildlife relocation center/hospital in our area. We always seem to find more. As someone else said, if your backyard is habitat - it's habitat. Killing the odd one you see out in the open doesn't mean your kids aren't still going to run the risk of being bit.
I keep our firewood and other stacks of materials well away from where the children play and have fenced the area off. Educating the kids and relocating potential habitats away from where my kids and other pets play has worked fine for me.
Every few weeks I'll check the firewood and other building material piles and bag any rattlers I see. I get maybe four or five a year.