Under Rayleigh scattering the sky will look greener and then yellower the closer you get to the sun, however with the sun high in the sky I don't think that's what is happening here. I think pollution is a more likely cause, especially as it seems to exist close to the horizon. Filters (polarizing/UV) may make the sky a darker, richer tone, I'm not sure they would change the hue, though I suggest you try filtering if you have one to hand.

The white balance in the images is fairly warm, which looks good for the skintones but will always give a yellower sky than a cool balance. To maintain the skintones you can always adjust the hue of just the sky by a selective colouring tool in post, or boost the saturation of the blue primary in raw conversion.

Hope you don't mind me editing and reposting the images, but here's the original first image, cooled down slightly:

And here's the original image with just the cyan tones shifted toward the magenta end of the spectrum in Photoshop:

And finally just to show you how far you can go with selective colour adjustments, here's the original image cooled down with the sky shifted and the grass made to look a little bit more alive!

Finally, the second image seems to display lens flare on the right hand side, is it a crop from a larger image? If not I don't know where the flare is coming from as you're not shooting that close to the sun. edit It's probably just something white floating close to the lens, thus totally out of focus.