If someone wants to seek refuge with Allah from more than one child, is it sufficient for him to wipe his hands over both children in one go, or should he do that for each one separately? We have not come across anything regarding that, so to be on the safe side, he should recite ruqyah and wipe his hands over each child separately.

If a person recites Al-Fatihah then blows into the water, there is nothing wrong with this. If reciting the Quran and blowing into water is done with the intention of seeking blessing from the saliva of the one who blows into it, this is undoubtedly prohibited.