The name of the witches' front organization is a clever case of Exact Words: there cannot be any cruelty to children if there are no children.
The Bittersweet Ending seems rather dour, in that the boy-mouse decides having a shorter lifespan isn't so bad since he can't imagine a life without his grandmother. Then one has to consider that he and Grandma will have full run of the Grand High Witch's castle, which will include access to her money-printing machine. There's hope that in the castle, they can find more formulas, potions and spells, possibly one to change the boy back and to extend Grandma's lifespan. If not, they'll find more ammo to use against witches rather than the mouse-making formula.
Grandma sets Mr. Jenkins on GHW as mild revenge, so that Jenkins vows to confront and sue GHW for changing Bruno into a Mouse. Technically, however, she doesn't lie; she tells him GHW changed his son into a mouse, which is true. She also warns him that if he tries to sue her, GHW will probably turn him into a cockroach. Not to mention the amount of evidence is lacking, given the magic and all.
Witches seem to have a flair for the dramatic, while wanting to avoid being caught. It seems to be more of a status symbol than anything; the Grand High Witch has the money for bail, and most of the disappearances can't be blamed on mundane issues. It also means that a witch wasn't responsible for killing the boys' parents; the witches would rather have children disappear in obviously supernatural ways rather than Make It Look Like an Accident.
There may have still been hope even if the witches had managed to get all the kids in England turned into mice. Once turned into a mouse with mouse-maker, someone is still A.) sapient, so the children would know to avoid the mousetraps unless they were already very dumb, and B.) able to talk, so they'd be able to tell the adults that they're not normal mice.
The dog poop smell that kids smell like to witches dissipates as the child ages, presumably during puberty. One of the changes that happens during puberty is that a different type of sweat gland activates, hence the B.O. the teens now have to deal with. Since it's stated that being dirty and avoiding washing hides the poop smell, it's possible that they stop smelling bad to witches because they start producing a naturally bad smell to conceal it.
After the narrator is turned into a mouse, he starts coming up with clever, shrewd ideas to aid in the plan to stop all the witches of England in their tracks (such as adding the Mouse-Maker in the kitchen rather than on the dining-room table in plain sight), and his grandmother exclaims that him becoming a mouse may have doubled his brain-power. From a biological standpoint (in as far as biology applies in a story like this), this would actually make sense. Since intelligence in organisms is often determined by the size of the brain relative to the rest of the body, it makes sense that having a human brain (or mind at least) in a much smaller mouse body would make for a sharply intelligent creature.
The likely fate of Bruno Jenkins certainly falls into this category, considering Grandmamma's opinion is that his horrified parents will take him home and drown him. Of course, this is merely what the main characters think has happened; it's never confirmed for sure. But even if you don't think this occurred, there's still the fact that Bruno, like the main character, has had his life shortened down to nine years due to being transformed into a mouse, and neither he nor his parents are aware of this. They may end up being in for a nasty surprise some years down the line...
And there's also Topsy the cat. If she stays with the Jenkinses (and Bruno isn't drowned), she might kill him, and if they do "get rid of" her like they said they would, where would she go?
At least on the witch's side, here is some horror; hotter climates wouldn't allow for them to wear gloves. They would stick out like a sore thumb. So it means that witches in, say, Cuba wouldn't be able to do a thing. GHW would be furious that some places wouldn't allow for that. Similarly, in some cultures it is not normal for a woman to be wearing gloves at all times, making it easier for witchophiles to notice and catch women who never remove their gloves.
Also if you are a witch, you may get fried just for showing up to the meeting and shooting your mouth off. And if you don't shoot your mouth off, or no one does, your leader will find other reasons to burn someone to a crisp.
As the boy lampshades when he encounters them, who were the three children that the Grand High Witch turned into frogs, to feed them to the seagulls? There's also the possibility that they aren't children, since the boy is just speculating, but them being thrown to the seagulls is still horrible even if they're normal frogs.
The Hotel Magnificent is going to have one hell of a PR nightmare on their hands once word gets out that a group of women turned into mice after eating the hotel's soup. That is, assuming they don't manage to cover it up somehow.
Remember how the witches use magic, hate kids, and want to get rid of them all? One could take a guess that the witches might have had a hand in the high child mortality rates of centuries ago.
The boy turns all the English witches into mice with their own potion, including the Grand High Witch, causing a mass panic in the restaurant. They get chopped up by the panicking waiters and chefs, but did they really get every single one? It's possible that some of them could have escaped in the chaos, especially since the restaurant doors were wide open because people were fleeing in terror from the swarm of mice. As the boy himself says, a mouse with the intelligence of a witch and the ability to get anywhere unnoticed would be very dangerous indeed...
A minor one, but can't the witches just wear a soft cap in between their wig and their scalp?
Given that the book/film probably takes place about 50s/60s, those probably weren't a thing yet or, if they were, the witches probably wouldn't be bothered.
