I don't mean to be presumptious - but if I may.....
From your posts, I get the impression that you are saving for this cruise. Please do not let it kill you financially for fear of what others will think.
For your little guy, get him two nice pairs of cotton dockers (something he can wear again) and a few nice collared shirts. Please do NOT waste your money on buying him a suit that he may never wear again. No one will judge you by your chidlren, and even if they do ... PHOOEY on them!
As for your daughter, let her get a nice sundress. That will work fine.
I don't mean to tell you how to parent, but if your kids protest, let them eat at the buffet that night. Why let the dress code affect your cruise?
As for you my dear, congratulations on the weight loss plans!!!! Kudos to you. I'd wait until closer to your cruise. You may get some nice end-0f-summer sales this year or next, where you can get a deal on some nice dresses. Go for staples like a little black dress that you an wear a few times, changing your accessories - jewellery, scarves, jackets, to get a different loook.
Don't waste your money on holiday-clothes, especially for the kids. Just remember, it all adds up, and you can put the money towards your next cruise holiday