(Strip Number 1267, Original Publication Date, 18 March 1988)
Panel 1: As Mike howls in pain after stepping on Lizzie's tooth, a delighted child proclaims that he found it.
Panel 2: As Elly puts the Kleenex back in the box, she tells Lizzie not to lose it again. Lizzie says she won't because she's going to put it in a secret place.
Panel 3: When Elly tells her not to make it too secret because otherwise, the Tooth Fairy won't be able to find it, Lizzie assures her mother that she will.
Panel 4: She then staggers her mother by reminding her that the Tooth Fairy is magic. This is because Elly fears getting caught being the Tooth Fairy and thus somehow leading Lizzie to travel the same dark path Molly is on.
Summary: Reading ahead, it seems to me that Elly seems to have made the very Elly mistake of taking steps to preserve a belief that might tend to destroy said belief. I'd originally thought "Elly can't understand what her kids believe" but
aprilp_katje is probably right to assume that Elly's ineptitude at preserving childhood faith is somehow Lizzie's fault for having a friend who got a half-dollar coin from HER Tooth Fairy.