I brooded my first ducklings in the tub last year and i'm going to do it again this year due to construction in the area where I wanted to brood them.
The smell does get worse as they get bigger.
I cleaned the tub daily last year and I used very old bed sheets that I replaced almost every day, depending on dirt level. I also used oxyclean to clean the tub every day.
Around 5 to 6 weeks they may be big enough to get out of the tub. Watch for signs of that :). Mine didn't actually get out but they were trying to jump up on top of the brinsea brooder so I knew it was time to move them.
This year i'm trying coop deodorizer ( sweet pdz) in the bathroom to help with the smell. I don't know if it will work but I hope it does.