Teaching comparatives without teaching grammar
Bigger, smaller, older, younger. The rule is simple, the exceptions are not, and neither is what a child needs first.
Comparatives look like an easy win. Add -er, or put 'more' in front, and you are done. Then a learner produces 'more big', 'gooder', 'more taller', and it becomes clear that the rule was never the hard part.
The hard part is that comparison is a relationship, not a word ending. A child has to hold two things in mind at once and describe the gap between them, and the language only makes sense once that mental move is comfortable. Teach the ending first and you get correct forms attached to nothing.
Start with the pair, not the word
Before any -er appears, the child needs two things side by side that obviously differ. Two shoes, one big and one small. Two towers of blocks. A photograph of themselves now and as a baby. The comparison should be visible enough that the meaning arrives before the language does.
Then you supply the sentence, they do not. 'This one is bigger.' Said while pointing, several times, across several pairs. You are not asking for production yet; you are attaching a word ending to a relationship the child can see. Production comes when they have heard it enough to try.
Opposites are the fastest route in
Comparatives are much easier to acquire in opposing pairs than one at a time. Big and small, old and young, fast and slow, tall and short. The pairing gives each word a boundary, and it also means every example teaches two things.
Books built on a contrast are unusually good for this, because the whole structure of the book keeps putting the two sides next to each other. Any story where a young character and an old one swap places, or want each other's lives, is running the comparison on every page — and running it about something a child cares about, which is what makes it stick.
Do not front-load the exceptions
Good and better, bad and worse, far and further. These are irregular, they are high frequency, and the instinct is to teach them early because a child will need them. Teach them early and you undercut the pattern before it is established.
Let the regular pattern become automatic first, accept 'gooder' cheerfully for a while, and introduce the irregulars later as individual words rather than as exceptions to a rule. A child who says 'gooder' has understood the system. That is a better position to be in than a child who avoids the structure because it seemed to have too many traps.
Recasting instead of correcting
When a learner says 'my tower is more big', the effective response is not a correction. It is to say the same thing back, correctly, as if agreeing. 'It is bigger, isn't it. Much bigger than mine.' The child gets the right form immediately after their own attempt, at the moment of maximum attention, and nothing has been marked wrong.
This is slower than correcting and it works better, because it keeps the child talking. A learner who is producing plenty of slightly wrong English is in a far stronger position than one producing very little careful English, and how an adult reacts in this exact moment decides which of those you end up with.
Comparatives are one of the first places where a learner gets to say something genuinely their own — this is better, mine is faster, I am taller than I was. That is worth protecting from being turned into an exercise.
Two objects, an obvious difference, the sentence supplied a dozen times before it is ever requested, and a shrug at 'gooder'. The endings look after themselves.