b and d, p and q, 2 and 5 - children often confuse these.
Brocolli or broccoli?
As a child, I could never remember if broccoli was one 'c' and two 'l's or two 'c's and one ‘l’. It wasn't until I started to check I was always correctly spelling it that I finally got it embedded in my memory.
Once you can't remember which it is, it can take a long time to get it correct. This is true of children too.
Reversals
Children often mix up b and d, p and q or even 2 and 5. If letter or number reversal is happening you need to work on just one of the pair they are confusing and really get it embedded in their memory before working on the other.
For example. With the letter pair b and d which is quite often confused by many children, I'd recommend working on b first. This pair is quite a common reversal with young children.
What will help children remember?
If they have they learned a mnemonic (memory aid) to help them, brilliant But if not ask your school what they use or make one up? Jolly phonics say bring my bat and bring my ball so when they say the rhyme as they draw/write it hopefully they will remember the stick bit (bat) first and then the round bit (ball).
When you think about it b d p q all have a straight stick bit and a round ball bit. It’s just the orientation that is different and which makes a different letter. As a child, it’s like looking at a chair. It doesn’t matter if it’s upside down, facing you or away from you, it’s still a chair. No wonder it’s so confusing for young children learning to read and write.
Always sit with your child for letter formation because sometimes they draw/write the letter shape and it looks correct but they are not starting and stopping in the correct place. If you are sat with them you can keep reminding them where to start. This is so worthwhile as they will be able to concentrate on what they are writing rather than how to write as they get older.
Moving on
Once the formation is improving, start adding b to words but make sure it's not a word with a d also as we are trying not to confuse them at this point. For example, do use words such as big, bug, bin, but, just not bad.
How can you help?
Little and often every day and your child will soon crack this.
Start with big movements with the whole body or arm then move onto small movements.
You can do it with bath crayons, chalk outside, big arm movements with a streamer/scarf/sock, walk the shape written with chalk or masking tape, trace a big shape with a vehicle, use whiteboards and pens, pencils, paint, with a finger in sand/salt/glitter, in the air, on someone's back…
The more they correctly form it the better as they are building muscle memory.
Remember once they start to incorrectly write it, it's harder and takes longer to rectify.
Hopefully, they will then be able to write something which makes sense...
Big bad Ben did not sit on a dot.
Rather than…
Dig dab Ben bib not sit on a bot.