I've occasionally had similar questions like 'are they yours', and it might be both because I (think :-) I look younger than my true age, and because of the tan of my kids. Also on one occasion someone asked me if I was one of the teachers since I was always spotted with one or two of my children when bringing to/picking up from pre-school.
I am Belgian, white-skinned freckle-faced with darkbrown eyes and fine light brown hair, in Turkey where I live they even call it blonde :-). DH is local and dark olive skinned with thick black hair. DS1 looks like a real mix and has middle brown hair, 'my' darkbrown eyes and quite a tanned skin. DS2 is dark blonde with the very same dark eyes and has a paler skin than DS1 but now summer has come he starts to get a darker complexion too. His features look the most like mine. Kids look like each other too.
You also find middle to dark blonde hair en bright blue, grey or green eyes in Turkey. But still I guess my kids look just that littlebit 'different' from others here, at least it's always being spotted.
My HB has been asked a few times 'are they foreigners'?, about our children and/or me. Mostly when I was not in the picture for a few seconds. He replied: 'indeed, but only half, mom is European' or sth. It may sound rude people asking if you'r foreign or a foreigner but it happens to me ALL THE TIME in this country. Sometimes you hear them say it behind your back but I sure do understand them). Whatever, I was quite annoyed about this kind of question but now realise it is just the direct way of curious Turkish people wondering where you're from. However, the often inquire because they positively interest in you (although exactly for being a foreigner) and/or for starting a social conversation. Still it's annoying, especially the frequency, but I guess I'll have to learn to live with it, that it 'll happen as long as I live here :-). But I DO hope that my children will not have too many annoying or embarrassing (in the meaning of rude by the questionneur) experiences in this regard. They ARE locals, yes mixed, and bi-lingual, but born and raised here.
I think it is important indeed that on the one hand you do not take the inquiries too seriously, and on the other hand try to give a plausible and polite answer (not defensive) as an example for your kids. My 4y old starts to understand more complex subjects and understands already that he is a mixture from parents of two origins, so he can also accept and think about our explanations about it. He likes to compare his darker skinned arms to mine too and finds it funny :-). And then I show him my freckles (which HE doesn't have) :-).
And they are both handsome (look 'different' and interesting to people in both Belgium and Turkey) - and oh my do they KNOW IT
. Hope that won't lead to arrogance or annoyment later in life :-). But they get to hear it all the time, also from complete strangers and they often get cuddled and kissed without 'warning', which my 4y old starts to resent! I assume random people will stop cuddling our children past 6 or so? Or maybe I should ask their IDs first