Just make sure you don’t put the swim nappy on til you get there - take them in normal nappies and change there. Swim nappies don’t hold liquid so if they pee in them, it will come straight out all over clothes/car seat etc! Padlock might be a good idea if you’ll need a locker, and snacks to keep them entertained while you’re changing - I find that’s the most stressful bit! Good luck - it’s really fun!
Swim suit and swim diaper then hold them in water. My daughter is too crazy to hold now so we have safe floatys that clip under her crotch area and we’ll see how that works. It’s definitely good to get them in swim lessons! My daughter learned a lot and even went under water. I wore a regular one piece. Each place for swim lessons will tell you what you need