Of course there is also the possibility of parent marisa being a shithead that deserves it, if the Kids are victims avenging something or if they are equally shitheads manipulated by anon to kill their own parents that would all depend of the anko.
Anon took away the father Marisa's hat and wear it. He then told the children that he's their father and he became weird (something grow under his feet/head) because of that uneasy Marisa's curse. The children then hurt their father with toothpick he gave them. When Marisa is about to die, he returned the hat.
Later, mother Reimu came back and saw two little uneasy yukkuri. The man then returned, wearing KoMarisa's hat and attached KoReimu's ribbon to his shoulder. He told Reimu that those uneasy yukkuri cursed her children so they must be punished.
Well gotta admit, while it's a typical "accesory steal" ploy it was rather original by playing it like "I'm a bit different because that weird manjuu cursed me, AVENGE ME!" and having the rest of the family falling for it.