DD1 has always been confident in water and in the toddler pool was ok to pick herself up if she fell over, DD2 didn't want a bar of it when i went with a friend but in lessons did quite well.
She started in November so before she turned 2 and she has now finished the 1-2yr old level, skipped the 2-3 and the first level of toddler lessons and is in courage 2.
Because she is under 3 i still have to be in the pool with her for lessons BUT as soon as she hits 3 i'm outta there.

Both of my girls can wear floaties and swim alone in the deep end, they can both use just a pool noodle to keep themselves afloat aswell.
In the toddler pool though they don't use any floaties, just run around like nutters.

Also if your worried you can always let the life guard know you have multiple children and you may need them to wrangle one if they jump out of the pool etc, most of them are pretty nice about stopping a little runaway

I'm sure they will be fine hun.