DD was in the water with DH for the first few months, then "graduated" up a level, and within weeks, she is swimming (dog paddle) by herself with the instructor in the pool there with her.

i really think how quickly they will pick up the skills will be dependent on the kidlets - i would guess your 2.5 year old will pick it up quicker than the 4 year old - and maybe it might be worth looking at getting the 11 month old in there soon too.

DD is a very over confident little girl, but she has picked up the safety stuff more rapidly now that DH isn't in the pool with her - hopefully similar happens with your 4yo