I think most fruit stones contain trace amounts of cyanide, but you'd have to eat a whole bucketful of apple pips to get any form of poisoning (it's that slight almond taste you get when you bite into one).
Nectarines. The stone fell apart and inside was an almond-like nut. Curious, we ate half each. Definitely tasted of almonds, but had a very unpleasant bitter aftertaste. Our tongues went a bit tingly and numb.
Some hours later dd had horrid stomach cramps.
I had meanwhile looked up nectarine pits and discovered that they contain cyanide, which is released by some complicated chemical process or by being exposed to air, depending on what site you're reading.
Anyway, don't eat them if you find them, do tell your kids not to eat them - especially small children as apparently it could only take a few to kill a small child (dd is 13). This goes for peaches, apricots etc.
DD's fine today, btw, and is having a lovely time telling everyone that her mum poisoned her