The boys will need a pair of decent long pants (not shorts or jeans) for the dining rooms at dinner. Otherwise, shorts and jeans are fine. They can wear polo shirts, no need for a tie at their age.
If you want to dress them up in a suit or whatever, there are always kids dressed up...but it sure isn't required or expected. One pair of dress shoes (or even black tennies), and comfortable shoes. We always pack too much....less is definitely better. Just have a light weight jacket for everyone...cool evenings are possible.
They ask that we not wear shorts to dinner, and that you "dress up" for Palo (the adult only restaurant). Other than that, it's very casual. Formal night can be as formal or informal as you wish...and you will see the full spectrum...just let the boys be comfortable! No point in making the kids hate dinner because they have to dress up so much. If you have a little girl who likes dressing up, she will have lots of chances.....