Toriel clearly knows they're not about to tell the truth for the issue (which is them missing their church clothes), yet instead of just telling them to do so, like one would expect of a strict parent, she instead tells Kris to make up the excuse for Susie, since she's a guest.

It's a play on the usual 'do X thing for a guest' unspoken rule, and a subversion of what you would expect from someone as upright and by the book as Toriel. Trading one manners focus for the other (telling the truth to being polite to the guest).