Sun block is not optional for the wee ones.

And a light shirt/pants/onesie would likely be okay to.

Make sure to bring a bottle, not water, to avoid dehydration

I’d still put them in a light onesie. They’re pretty small and they don’t have much thermal mass, so they may get cold despite the weather. Keeping them in shade like you’re suggesting is a good idea.

And like others have said, have lots of formula/milk for them to drink.

If they’re having a bad time they’ll probably let you know, so be ready to head indoors if they get cranky.