In Dodie Smith's novel, the kidnapping of the puppies takes place off-stage and is only discovered after the fact. Bill Peet wisely realized that this event held great potential for conflict and suspense, and he uses it to introduce the remaining villains Horace and Jasper. The sequence also gives Nanny a chance to shine. Her dialogue with the crooks at the front door shows that she's no fool and her physical resistance once the crooks enter the house gives her heroic stature.
When I see sequences like this, I'm always puzzled as to how animators are assigned. For shots 2 through 8.1, Horace and Jasper are sitting in their truck discussing the job to come. Eric Cleworth and John Sibley share these shots. I would think it would be simpler to have one animator take care of everything or have each animator assigned to a single character. Cleworth and Sibley both have shots where they animate both characters, so it's not like one of them was incapable of some part of the sequence.
Horace and Jasper are very economically introduced. We quickly learn that they're crooks with a record and that Jasper, the tall thin one, is the dominant one of the pair. When they get to the door, it's Jasper who delivers a line of blarney intended to get past Nanny, and when she firmly resists allowing them to enter, it's Jasper who forces his way in and then traps Nanny long enough for Horace to grab the pups.
John Lounsbery's animation of the pair at the door is wonderful stuff. Jasper has a very flexible face and Lounsbery makes the most of it while Jasper attempts to con Nanny. Shots 34 and 35, where Lounsbery animates Jasper talking at the attic door also show off some great facial expressions. In shot 30, Lounsbery animates a terrific walk when Jasper goes up the stairs. Jasper's proportions are very odd; he's all arms and legs with a hunched over body. His walk is distinctive and funny, looking like a very odd spider.
John Sibley also gets some very good Jasper shots. Shot 8.1. clearly establishes the relationship between the two bad guys. Sibley animates Horace and Jasper approaching the front door in shot 12, doing an excellent job on Jasper's walk. Sibley animates Jasper pulling Nanny's hat up and releasing it in shot 29.1. He gets a bit of Jasper at the attic door in shot 34.2. Sibley's handling of Jasper is as good as Lounsbery's, but he never gets a chance to build up any kind of performance with the character because he rarely gets two shots in a row.
Hal King continues to animate the puppies for the few shots they have in this sequence. Where Ollie Johnston animated a lot of Nanny in earlier sequences, here Cliff Nordberg and Don Lusk inherit her and do a fine job of maintaining the drawing and personality of the character. This sequence is Nanny's big acting scene. In fact, in terms of range and emotion, it might be the biggest acting scene of any of the human protagonists and I'm surprised that the nine allowed someone outside the inner circle to animate it.