Question is as follows.
There are two parents and three children.
"Set, A, must contain at least one parent and two children."
My interpretation is that A has to have at least one parent, and only two children
But, my I guess it is possible to understand it as "at least one parent and at lease two children"
which one is right interpretation?
The more probable reading is "at least one parent and (at least) two children". The other reading would have been rendered "at least one parent and exactly two children".
That said, if you are unsure, contact the author of the text. We can't read their mind, and they might not be native speakers and so could intend the "wrong" interpretation, since that's the way their mother tongue works, or that's the way they had been taught English works.