Don't get the whole conversation going, but based on Maricha's speech, I believe they were so hungry (because in some of Maricha's bubble, she mentioned about "Gohan-san/Mister Food") that they went out to go "hunting", and the rest is history. Meh, obey your parents, kid.
It seemed that the father Marisa was eaten by crows as well.
*Crow*Yuu...nYuguu...Yuu...yuu...*Shiver*Catching illness can't take it easy...
Reimu wanted to focus on babysitting in the first place...Yu...
Yupii...Mister Birds are nasty...*Lick*so munch-munch lots, don't take it easy and pull through!Mommy has brought you a lot of Mister FoodAfter getting to the outshide
[We're] bullwied by Mishter Birdz noje...*Trikcle...*Maricha... and little shish ash well...
Couldn't bwear Mishter Emptchy-Emptchy noje...Why is little one at the outside and woundeddd!!!Mommwie...
[Maricha] wantsh lwick-lwick noje...Yunyahh!!