YABU for your reasons, but YANBU for confiscating them.
I hate Horrible Histories. They are the educational equivalent of junk food. Children will do themselves no favours by reading them. They reduce a perfectly interesting subject to burping, farting, murders and toilets, and children who are introduced to history through them have no incentive to continue onto books that don't sex up the subject but do teach them stuff worth knowing.
I suppose there is the odd child who goes onto become a history buff, having been introduced history via Terry Deary's books (which, by the way, he himself describes as "entertainment" rather than "educational") just as I suppose there is the odd child who goes onto like vegetables having been introduced to them via crisps. However, I suspect there are rather more children who can't be bothered because they have never been taught to find history or other subjects interesting themselves, and don't learn to persevere without a reference to flatulence every other page.
It pains me that HH have even found their way into my DD's school when she learned about the Romans last year. She knows absolutely fuck all compared to what I and my classmates knew at her age (30 years ago) on anything factual. Compared to the books I got to read, the stuff made available to her by the local library and the school tends to assume that children have a low boredom threshold. It is all very low-content, high-entertainment, and it makes for low-content, highly boring children.
As for Deary's comments on libraries, they are indefensible. He says we wnt things for free, assuming that libraries are free. Of course they aren't, they are paid through rates, which people either pay directly or are indirectly factored in as part of their rents charged by landlords who do pay rates.
OP, burn the fucking books!