I'm just going to give a bit on swimming lessons first, even though its not the answer you want, it can provide some answers to the "when". Then I'll talk about some of the things we do in the classes which you could do on your own, as lessons are a good guide.
I live in Australia, where its very common to start taking your children to swimming lessons from 6 months. Even though obviously they won't be able to swim until later on, its very important to teach them about water, water safety, understanding to hold their breath when they do go underwater.
I have a 2yo who has just finished her second term of swimming lessons (we started her at 12 months, intended to start earlier). She can now swim with a floating device on her back (not for long and I would not be much more than an arms reach away from her.
The classes for 3 year olds+ I believe they start to learn to swim on their own.
As I mentioned we started our daughter at 12 months, so I'm less aware about the routines they do in the youngest classes. Some of the beginner things she's done from 12-24 months are:
- Chin in the water.
- Left/Right ear in the water.
- Blowing bubbles (this is really good to teach them about not breathing in, but out and can lead to holding their breath)
- Lying on their backs, head on your shoulder and kicking.
- In a shallow pool, trying to pick pool toys off the floor, you can assist them a bit, but teaches them to put the head into the water on their own.
- With a floating device on their back, running to fetch a floating toy. (Support them at first, but they can do this on their own fairly easily).
- Jumping into the pool - standing on the edge, touching their toes then jumping to you. (Let them fall in the water, don't catch them straight away. Also assist them to climb out of the pool on their own)
- Underwater swims, with someone else, guiding them underwater to the other person (Teach them to hold their arms out in front and hold their breath).
With all these things, as they get better you can support them less, and get them to do more on their own.
I know you said all under 6 years old, so some of the activities may be too basic for them. I'd really suggest getting some sort of floatation device, (floaties, lifejacket) and assisting them until they feel comfortable on their own. But I don't have the experience with older kids yet, so its just what I'd imagine would be good.
I should also note as I did in my other answer. Learn CPR for kids!