Attempting to boast an ability to kill a human (that would kill the Marisa), I guess this Reimu is trying to take over the cardboard house. Whatever the children are saying must be contrary to the mother's opinion of her exaggerated story; or, they're goading/cheering their mother on to fight the Marisa. Not to plan.
They're agreeing with her, then marisa claims she is a liar and the children say kill marisa but the mother knows she can't kill a marisa and hesitates at which point the children doubts her, the larger reimu says she'll show them her power.
when there's words "i'll show you my power" like these there's gonna be a fight...
i hope Marisa's family is safe
and i'm not sure that i trans it good enough
is it like Marisa's has seen mister's power be4 but Dosu(big size)-Marisa came to save her and dead instead her too right? that's why Marisa doesn't believe this shit story right?
(trying to study japanese but still lot more to go ^^")