This should not be a difficult task at all if she is getting good phonics instruction. With well taught phonics she should be able to spell most of them (and hundreds more words) by the end of Y1
There is nothing special or mystical about these words, nor are nost of them uniquely difficult to spell.
This is my analysis of the 100 HFWs sorted into 'simple' code and 'advanced/complex' code. Ideally they should be covered when the code they contain is learned.
They should be 'learned' by matching the phonemes they contain to the letters which 'spell' them. Dictate the word, child identifies the phonemes it contains, child writes the letters representing each phoneme in the order in which they are said in the word (ideally, saying each sound as she writes it), child checks word is correct by sounding out and blending exactly what they have written.
Look, Say, Cover, Write & check is not a good method.
It is ridiculous for schools to make a fuss about children learning these particular words in isolation from normal phonics teaching. There are many hundreds (not to say thousands) of 'useful' words in our language; their 'frequency' is irrelevant once children start to write creatively and exploit their own vocabularies.
100 HFWs from Letters & Sounds. These are grouped according to the 'code' they contain.
Simple code:
and, a (?), in, it, on, is, at, his, but, that, with, can, up, had, this, went, not, then, as, mum, them, dad, big, it's, from, him, get, just, got, put, if, help, an, children
She should be able to spell all of these already with the phonics she (should have) learned in YR
Advanced code - not unique Letter Sound Correspondences; could be gruoped with other words containing the same correspondences:
She should be learning all this 'code' in Y1
He, she, we, be, me
see
was, what,when
you
they
I, I'm
for
all, call(ed)
my, by
her,
there
out, about, house
have
like, time
some, come
so, go, no, don't, old
little
do, to, into,
down, now
looked, look
very
will
back
came, made, make
their
your
could
too
day
off
asked
saw
Advanced code - unique or very unusual LSCs
are, of, said (?), one, people, here, were, the (?) oh
Miscellaneous abbreviations:
Mr, Mrs
These 'may' need a bit of extra work, but still (apart from the 2 abbreviations) mapping the letters to the sounds they spell.