It's basically some Mom common sense :)
You can use bucket/fishing type hats for their little heads, faces and necks.. this will easily help block the sun off the top of their noggins!
You can also put them in light weight clothing.. pants and tees that are breathable so that air can flow in/out, but so that the sun cannot beat down on them :)
You can and should try to use breathable blankets over strollers any time you are out n about and the canopy does not cover the baby completely!
Also remember the feeeeet... never forget the feet!