I wouldn't be letting my kids play unsupervised anywhere where a Caution sign was needed (major road) and since I'm not into supervising kids playing, they wouldn't be there, they'd be on their own street or in their or friends' back yards. I think those are stupid signs, like the Baby on Board one. I do adhere to the playground and school ones, though.
If your kids are out playing near cars, either be there with them or they should be old enough you can trust them in your absence. Either way, I don't pay attention to the signs. I don't speed through residential neighbourhoods and I do look around, but if your kid comes dashing into the road to get a baseball, I don't consider myself responsible if he gets knocked down. Either teach them road safety or keep them in their backyard.