I find it really hard to remember whether each child has encountered the word before!
No need to worry. U'll find out which words are tricky for your children by them stumbling over them, because they are not entirely decodable - because some of their letters or graphemes like 'ow' can be pronounced in different ways.
A few years ago I had identified all the tricky words in each ORT book, but can't find them now. Some contain far more of them than others.
Among the 100 most used English words the following are not entirely decodable:
he, of, the, to, was,
all, be, are, have, one, said, we, you, by, my, call, before, come, could, do, down, into, look, me, more, now, only, other, right, she, some, their, there, two, when, want, were, what, where, which, who, your.
In the top 300 there are these as well (although some are derivatives from the above):
after, another, any, asked, bear, book, can?t, coming, couldn?t, don?t, ever, every, everyone, eyes, fast, find, four, friends, gone, good, great, grow, he?s, head, I, I?ll, I?m, key, know, last, laughed, live, lived, looked, looking, looks, many, most, mother, Mr, Mrs, never, oh, once, people, plants, pulled, put, ready, river, small, snow, some, something, there?s, thought, through, took, town, very, wanted, water, work, would.