Most parents who ask this are not really asking about swimming. They are asking about water, and about a child who heads straight for it without checking whether anyone is watching. For a family whose child wanders, a pool or a drain or the sea stops being a nice afternoon and becomes the thing you think about every time a door is left open. If that is why you are reading this, you are asking the right question, and the answer is that water safety is a reason to start sooner rather than a reason to wait.
The honest part is that ordinary lessons often fail these children, and almost never because of the child. A standard group lesson is eight children, one instructor, spoken instructions across open water, a different order each week, and a pool hall with the acoustics of a car park. For a child who processes speech slowly, or who finds echo and splashing physically painful, nearly every element of that is working against them before the swimming has even started. A child who melts down at the edge of a group lesson is usually not refusing to swim. They are telling you the room is unbearable.
What changes things is nearly always the same short list. One to one, so nobody is waiting their turn in cold water while their attention drains away. The same routine every week, in the same order, so the lesson becomes predictable before it becomes difficult. Skills shown rather than explained, because watching is easier than holding a sentence in your head while trying to stay afloat. And steps small enough that the swimmer succeeds at something every single session. For some children the first month is spent sitting on the steps with their feet in the water, and a teacher who understands this will tell you that is not a wasted month — it is the foundation everything else is built on.
Much of this holds for other profiles too, with different emphases. A child with Down syndrome often needs the foundations — floating, body position, breathing — given far more time than a syllabus allows, and skipping them to keep pace with a group tends to cost more time later than it saves. A child with ADHD usually does better with short, clearly bounded steps than with a long spoken instruction they are meant to retain while also staying afloat. A child with global developmental delay needs progress measured against where they started rather than against an age chart, because a small step held confidently is worth more than a big one that has to be retaught next week.
If you are choosing a teacher, the useful signals are not certificates on a wall. A good one asks about your child before they talk about strokes — how they communicate, what frightens them, what a hard day looks like. They are willing to spend a whole lesson achieving nothing visible. They notice distress early and stop, rather than pushing on in the belief that your child will get used to it. And they will tell you when they are not the right person, which is the single most reliable sign that they are honest about the ones they are. Be wary of anyone who promises a timeline before they have met your child.
One last thing worth saying plainly. Progress in this is rarely a straight line, and a child who swims a width in June can refuse to get in at all in July, usually for reasons that have nothing to do with swimming. That is normal, it is not a sign the lessons have failed, and the right response is to go back a step rather than to push. Children who are given that patience get there. It just takes the number of weeks it takes, rather than the number a syllabus printed in advance.