You ask 'when did this happen?'. If you're 35, I'm surprised that you've even heard the phrase 'half caste', other than in old movies. Back in the 1970s, when I was teaching, it was a disciplinary offence (possibly leading to sacking) for any teacher employed by Sheffield LEA to refer to a child as 'half caste'. 'Mixed race' was the only acceptable term even then.
In the 1980s I attended a course, organised by Sheffield LEA, on racial awareness. I asked the two gentlemen running the course (who were both of Afro-Caribbean descent) to provide me with a politically correct word which could be used to refer generically to all 'non-white' children. I was told, quite emphatically, that the only acceptable word was 'black'. I asked whether this included children from, say, Chinese families and was firmly told that I should only refer to them as 'black'. (I later reported this to one of my pupils, whose family ran the local Chinese takeaway. Her response was less than polite!).
So your children are definitely 'of mixed race' and, quite possibly 'black'!
Chris