V.W.
We are Lutheran, and yes some people enclose cash in a card for the Confirmand and/or First Communicant.
My son was confirmed/had first Communion 2 years ago and received cards, cards with money, and even a card with a gift card for a restaurant. Our daughter will be confirmed this Sunday, and we are giving a keepsake box with engravings on the top to her fellow first communicant.
I don't know what is "expected" in Roman Catholic circles, but a card with cash would not be frowned upon, I am sure.