When my kiddos were really little, we used the Swimways 1-2-3 system to help us teach them to swim. Swim lessons were also attended, but, the "Power Swimmer" life vest allowed them to gain confidence very quickly, and consequently they became good swimmers at a very early age.
You can see the Power Swimmer vest and watch a great video here:
I am in no way connected to this company. Just a fan of what they offer.
Keep a close eye on those precious little ones!