I'm pretty sure whatever fuwa fuwa is killed (or maybe just knocked them out? Could have been some kind of sedative) the kids, then their parents accidentally started to eat the bodies before they realized.
It looks like they just fermented along with the fuwa fuwa, you can sorta make them out when their parents are holding them in the 3rd to last panel (though Marisa is without her hat).
There's very little to guess from what the hell fuwa fuwa is but I would guess that it's fermented fruit/berries. Alcohol would account for kos dying and feeling of warmth (Well, for humans but who knows if the artist even knows about why alcohol makes you feel warm, much less if he gives a damn about why it wouldn't work for yukkuris)