Idioms are the last thing a learner gets, and the first thing they hear
A child can know every word in a sentence and still have no idea what it means. That is not a vocabulary problem, and vocabulary practice will not fix it.
A child who has been learning English for a year can read the sentence 'put yourself in her shoes' and understand every single word in it. Put. Yourself. Her. Shoes. All five are words they know, and none of that knowledge helps, because the sentence does not mean what its words mean. This is the moment where a learner who has been making steady progress suddenly feels like they are back at the beginning.
English is unusually full of these. Not just the famous ones a textbook lists, but the ordinary furniture of adult speech: hang on, give it a shot, keep an eye on him, that went over my head. A learner hears these constantly and cannot look them up, because looking up the individual words produces nonsense.
Why the usual approach does not work
The standard move is to teach idioms as a list. Here are ten idioms about weather; here is what each one means; there is a matching exercise on the next page. This produces a child who can pass the matching exercise on Friday and has forgotten the whole set by the following Thursday, which is roughly what happens with any vocabulary learned as a definition rather than as an experience.
The reason is that an idiom is not really a word, it is a compressed situation. 'In someone else's shoes' is not a synonym for 'empathetic'. It carries a specific picture — you, physically wearing the footwear of another person, walking a distance in it — and the meaning comes out of that picture. Teach the definition and you have handed over the meaning while throwing away the thing that makes it stick.
What actually makes one stick
Three things, and they are all the same thing from different angles: a picture, a situation, and a repetition. The learner needs to see the literal image the idiom is built from, encounter it inside a situation where the figurative meaning is obvious from context, and then meet it again several times without being tested on it.
This is why idioms are learned so efficiently from stories and so poorly from lists. A story supplies the situation for free. If a character spends a whole book literally swapping shoes with someone and discovering what their day is like, the expression is not being defined, it is being demonstrated — and the child arrives at the meaning themselves rather than being handed it. Meaning a learner arrives at is dramatically stickier than meaning they are told.
The literalness matters more than it sounds. Most idioms started as a picture and then wore smooth from use, until native speakers stopped seeing the image at all. A learner has an advantage here: they still see it. Anything that puts the original picture back in front of them is working with the grain of how the expression was built.
Do not explain it the first time
The instinct, when a child looks blank at an expression, is to stop and explain it. Resist that on the first encounter. Let the sentence go past. If the surrounding situation is clear, a learner will usually build a rough guess, and a rough self-built guess is a better foundation than a precise donated one — it comes with a memory of the moment attached.
Explain on the second or third encounter, when they already half-know and are looking for confirmation. At that point an explanation lands on something instead of into nothing. If the guess was wrong, correcting it takes ten seconds and they remember the correction, because being slightly wrong about something you worked out yourself is memorable in a way that being told is not.
Teach the frequent ones, not the colorful ones
Idiom lists in materials skew heavily toward the vivid and the rare — raining cats and dogs, kick the bucket, bite the bullet. These are fun to teach and almost useless, because a child will hear them a handful of times a year while missing dozens of instances of hang on, sort out, and I'm not sure I follow.
The expressions worth spending time on are the boring, high-frequency ones that adults use without noticing. They are harder to make a lesson out of, which is exactly why they get skipped, and they are the ones standing between a learner and an ordinary conversation.
Watch for the ones that are quietly cultural
Some expressions assume knowledge a child may not have. Anything drawn from baseball, from American school traditions, from a shared childhood text — these are not language problems, they are background-knowledge problems wearing a language costume. A learner can master the grammar of a sentence and still be locked out of it.
Expressions built from universal physical experience travel much better. Feet, shoes, hands, doors, weather, food, sleep. A child from any country knows what it is to put on the wrong shoes. That is why the shoe expression is a good early one and something like 'out of left field' is not, whatever the level of the learner.
The reason idioms feel disproportionately hard is that they are the point where language stops being a code to decode and starts being a culture to join. Every one a learner picks up is a small piece of belonging, which is why the day a child uses one correctly and unprompted is worth more than a test score.
So slow down on them. Let them be met before they are explained, choose the ordinary ones over the exotic, and lean on anything that shows the picture underneath. The list on the worksheet is not the work.