They really shouldn't be that weak when there were... what, 3 adult Yukkuris and their foes only have one, and the rest were kids. And I really can't understand those glass windows, so fragile. But, it's his story, and I enjoy imagine the rest myself, so yeah, whatever.
Alices break into the house; there might be more than one adult, you might only see one on screen. (Actually, I think the one butting the Reimu might be an adult too, just a smaller one.) The Patchouli faints immedeately; now we're down to two adults and (by my count) five koyukkuri. The five ko's appear to be much smaller than the Alice ko's (possibly newborns).
I count four full grown pet Yukkuri. I see one full grown invasion Alice, two Smaller adult Alices, and three Ko-Alices. Now answer me this, why did none of the Alices die?
I'm tempted to think pet yukkuris are less capable of protecting themselves from home invading yukkuris, and even if those other two adult Alices are smaller, they're probably more than a match for the two Marisas and Reimu. And the fourth pet yukkuri is a Patchouli. She probably fainted the moment the invasion began, so it's really just the three pets versus a full grown Alice and the two smaller Alices.
If I had pet yukkuris and I came home and found a group of Alices had broken in and raped them to death...I'd lull them into a false sense of security by giving them food and promising them a yukkuri to refresh with. And once they get all excited, I'd staple their penipenis together and observe what happens next.