oh my goodness don't please please take my word for anything, I've told you I work in SEN....
fwiw (and if anyone else toddles in then disregard this lot please) it's developmental. ds1 got the idea of number bonds halfway through yr 2 (late) and ds2 gets it now - after 1.5 terms in reception (early i think) so perhaps late reception to Y1 is more average?
again we do/did it with heaps of 'stuff' - get ten 'whatevers' and put all ten in one heap. move one on to a mat / different carpet / do boats into the bath, count how many in the 'new heap' and how many in the 'still here' heap. I do this at a point soon after they have a reasonable grasp of addition, using random numbers up to 10, without the desperate need to count every digit on their number line or fingers. erm if that makes sense. perhaps more a case of 'try it anyway' and see what they make of it. Chances are it is a game, moving things from here to there (fabulous way of getting stuff tidied back into their boxes) which means it's fun, which means they learn it without really realising it.
And when that translates into the 13 and 15 thing, erm I honestly don't know, my lot are far too literal for that.
With my own dc, and at work too, we use 'zones' to teach HTU place value. Colour coded mostly. columns of each colour, ask the child what colours make most sense to them. some pick traffic light colours, ds1 gets them 'warmer' as they go higher, thus hundreds is red, tens orange and units yellow. well it makes sense to him anyway at home we use lego, duplo and megablocks to do it, understanding that ten lego bricks = 1 duplo brick. at work we have the 'Base ten' blocks. physically put the blocks on the different coloured columns to show the number in each one. Essentially it is an abstract concept to be learned by children who have little abstract thinking experience so the answer is to make it as concrete - well as practical as possible. single malteser, bag of maltesers, box of maltesers would be my idea of a cool lesson
Mainstream primary teachers, where are you all when we need you???