On our 7 night my hubby wore his tux to both formal and semi-formal nights. On formal night he wore a bowtie/cummerbund set and a wing tip shirt. On semi-formal night he wore a shirt collar tux shirt (regular collar but pleated front and french cuffs) with a black silk necktie and looked really good. This combination was suggested by the men's store fashion consultant (what a trip shopping for men's clothes is!). He didn't take a suit but he did take a sport coat to match with khakis for lumiere's and palo's (we didn't go to those on formal nights). He gets to wear a tux so infrequently that I really was glad he wore that and not just a suit, and since it is his own tux and not a rental, it fits him absolutely perfectly. When we were shopping for one it was amazing how different the various cuts looked on him. If your husband already has it, he should wear it! (and can wear it for both formal and semi formal nights, we did see others wearing their tux twice without the change in neckwear). I think your boys would look fine in suits but if you want the whole family to match and be dressed to the nines then I'd get them tuxes too.
Lisa