It sounds like you've got the casual nights covered pretty well. However, Celebrity (at least on thbe 7-day and longer cruises) tends to be a tad more formal than other mainstream lines. The boys will want at a minimum coat and tie for formal nights, or they will stick out. They won't be the only teenagers sans formal wear, but teenagers don't like to stick out among their peers, so formal wear, a suit or at least a sports coat and tie will help them "get by" for formal nights. While informal nights also "call" for a tie, you'll find a large minority of the folks just wear a sports coat without it.
Celebrity publishes a good guideline for dress on the website. Here's a cut-and-paste:
A gentleman's choice for casual nights includes a sport shirt and slacks, while women will be comfortable in a pants suit, skirt and blouse, or casual dress.
On informal nights, men are requested to wear a jacket, shirt and tie, while women may want to wear a suit or dress.
For the formal nights, including the Captain's Welcome Aboard Cocktail Party, dinner on the second night, and the Captain's Farewell Dinner held the next-to-last night of the cruise; both men and women may prefer more dressy attire, such as an evening gown for women and a tuxedo or dress suit for men.
They also have a reservation-based casual dining venue, where you can dress casual each night, but it's a limited menu. The Mercury is a very nice ship and you should have a great time!
