Are you worried that your 6 yr-old can't swim and won't take their feet off the bottom?
Don't be!
We took ds1 swimming loads when he was tiny, and he had regular lessons from age 5. But until he was 6.5 he simply wouldn't take his feet off the bottom.
He's 9 now and has just passed the highest of the basic ASA swimming levels (Level 12), including racing starts for all 4 strokes, tumble turns, etc., etc! He has great style, speed and stamina, and is a far better swimmer than I ever was.
He's not going to be an amazing club swimmer or anything, but I am very proud of his achievement. And it just goes to show that being a slow starter isn't necessarily a problem, as long as you persevere and keep trying.