I might have enjoyed this when I was ten, but I am not the sort who can get into juvenile fiction much.* I abandoned Harry Potter midway through the second book too. The anachronisms annoy me, not that they should. I majored in history, focusing a lot on medieval history and the jumbling events just reminds me that the actual history or literature from the period are much more interesting to me. Frankly, Arthurian legend has always been boring, including Malory.
The other sections of the book are a lot different from the first section. You may want to skip ahead and consider looking into the other sections. Like you, I thought the same of the first part but I got into the later sections a lot more.
Indeed, as was mentioned in the Anachronisms thread as well as in the podcast, the voice of the narrarator grows up as Arthur grows up. He's just a kid in Sword in the Stone, but once you get to The Witch in the Wood and beyond, you'll find the tone changes significantly.
I'm not sure if it's because I'm on the younger side of people in this group (only just turned 22), or if it's just a character trait of mine, but I still can really enjoy and get into things that are aimed at children. I just thought the first book was fun, nothing that really needed to be serious or anything, just kind of like...play. Or maybe it's just because I LOVED The Sword in the Stone cartoon when I was younger, so there's the added bonus of finding out it was based on a book and actually reading the book. Kind of a nostalgia thing.
Yeah, I found the beginning to be very boring too. It wasn't actually until the Candle in the Wind when it became more reflective and when there was actual conflict that it became interesting.
So far out of the four books, the only I got 'bored' at was The Candle in the Wind. During chapter three, White talks/describes the Middle Ages. He goes into different details that it distracts from the story. I was tempted to skip this chapter, but did finish it. So far the next two chapters did pick up the pace.
Listening to the audiobook version--the narration is so good, it was hard to get bored. The only place it slowed down for me was the Book of Merlyn, because it seemed to become too didactic.
* Nothing derogatory is meant by this.