Allah has appointed the recording angels to record every deed—good and bad—that a person performs. These noble scribes bear witness to human actions, ensuring that every individual will face their records of deeds on the Day of Judgement.

When Allah commanded the angels to prostrate to Adam, He included Iblis in that because, even though he was not one of them, he was imitating them and doing deeds like theirs.

Seeing the angels and the jinn, in a form other than their true form, is possible. As for suggesting that only children can see that, we do not know of any text which speaks of that. This is a matter of the unseen, and we cannot say anything about it except on the basis of evidence, and there is no evidence to deny or affirm that.

There does not seem to be anything wrong with this phrase, and there is nothing to dictate that one should be strict about it or tell off anyone who says it. But saying “May the angels pray for forgiveness for you, and the like,” is more appropriate and is better.