Thank you so much everyone - it has been really interesting to read all your replies.
I totally agree with people who have said that swimming is an important life-skill. I think it's really important that children learn to swim. But not necessarily to be able to do all the strokes for example - unless it becomes a passion for them.
I should have explained that I do take them swimming very regularly. They love going and so do I. When younger they both used to be scared of putting their faces in the water, and really really didn't want swimming lessons, but have progressed to the piont where we love to have 'underwater meetings', and they seem to like swimming underwater more than on the surface. But I am not really teaching them the strokes... I am aware that my swimming might not be in very good technique.
With a view to starting lessons, I had them 'assessed' today. Ds (7) is stage 3, and Dd (5) is stage 2. I think they are probably behind children who have been going to swimming lessons regularly, and was feeling a bit guilty about that. But when I looked round at the lessons, my heart just sank. They don't look that much fun, and also (selfishly) I think that sitting in the reception with one child waiting for the other child to finish his or her lesson is a lot less fun than all being in the water together.
I also just feel slightly sceptical because I hear from people that children sometimes make very slow progress with their lessons. Why is that? Maybe it's because some people really do need an awful lot of lessons to learn to swim. Or maybe the lessons are just going on (and being paid for) while the child is just slowly growing and getting stronger anyway - i.e. the lessons are not really needed, and the 'progress' is 90% about just getting older. Or maybe the lessons are just not that good near us... I don't know!
Maybe I will take some advice from here and see if I can get a few private lessons instead of signing up to the weekly group lessons...