Have lots of fun in the water, as others have suggested, then as seems to be the complete opposite of what the mn opinion seems to be, pay for lessons. Sorry.
Unless you have a very solid technique, although you might get them to doggy paddle, you really might struggle to get them to swim well well. Generally, to be a strong swimmer you do need a good technique.
My dd1 is 7 and her technique knocks the spots off mine. I was never taught strokes properly. She's not a fantastic swimmer by any means, but because she's being taught, is pretty good. She can swim a fair distance, has a reasonable breaststroke and excellent back stroke, a passable crawl and is learning butterfly. That's after bog standard local pool lessons, btw, nothing special.
We swim for fun regularly as a family too. Get them moving, definitely, but I'd really recommend consolidating what you do with lessons at some stage. They might even want to if their friends are: dd1 loves getting badges too.
Neither of mine wanted lessons but were made to at three. Dd1 now can't remember not being able swim