It's Elly telling Mike to knock off the trying to peek crap.
Panel 1: Elly does isometric exercises as she confronts Mike and Lizzie with a semi-authoritative "Okay, you two....listen up!"
Panel 2: We shift to a fat rear wiew as she tells guilty-looking children that someone has been snooping around all the clostes and in the crawl space looking for presents.
Panel 3: We see her from their silhouette's perspective as she tells them they aren't going to find any because Santa only brings things to good children on Christmas Eve.
Panel 4: As she walks away, she reminds them that he knows who's been naughty and nice.
Panel 5: We see Mike in close-up as he thought-bubbles "NUTS!!"
Summary: First off, Mike did a lousy job covering his tracks so even an idiot like Elly could catch him. Second, she should have given them this lecture first so as to prevent them from doing so in the first place. Third, John probably knows of a great hiding place that she'd never think of if she lived to be a million; when he reveals it tomorrow, she'll unhinge her jaws and yell that this was all his fault for not telling her.