Yes, a pair of nice jeans with a polo shirt would be fine for your boys. Just make sure that they don't have rips or holes or are excessively baggy (what the kids call "fashion" these days
). If it's your first time cruising with your kids, it's not as formal as may think. I thought that it would be dressy every night, but the rules always relax. I even saw a couple of young girls wearing skirts with flip flops in the dining room.
Hope this helps and happy cruising!