Anyway, it's a trick that eluded my 5yo initially. She had a page in her workbook about adding 10 and I watched her using the number line to laboriously count forward, every time. She just didn't 'get it'.
This lesson is for children who need help in 'getting it' - understanding that they can just increase the 10s digit by one, without affecting the units.
It is useful to have a basic grasp of place value first. Luckily, Francesca had learnt all about tens and units in Reception class at school. Once you can talk in terms of tens and units, it becomes easier to develop the concept into addition and subtraction of 10.
It's also necessary for the child to be able to count in 10s - to know that 30 follows 20 or that the correct sequence is 70, 80, 90.
Once those principles are in place, you can move onto adding 10 to any number.
I told Francesca that soon she would know a trick to enable her to add 10 to any number at all, just in her head, in a second. Hopefully she was impressed.
So, we started with some random numbers between 0 and 89 on a sheet of paper. I asked Francesca to add 10 to each of them and write down the answer next to each number. I put 10 dots at the top of the page so that she could use them to help her with counting on. Which she did. All the answers were generated using the slow, inaccurate counting-on method.
Then we re-wrote the numbers in a list, showing that 42 became 52 and 15 became 25 etc.
Then we used a free online abacus resource:
It allowed us to represent the numbers as so many tens and units.
Then I asked Francesca to look at the result of adding 10, and got her to make the new number by physically (well, on screen!) adding a bead to the tens column. It was starting to really help, but she was losing concentration by this point (and was really much more keen on the idea of just adding lots of beads to see if she could fill up all the spikes) so we gave up for the time being and I let her have a play. I reckon if I come back to this concept another time, she'll totally 'get it' and we can move on to subtracting 10, or even working with numbers over 100.