I remember my parents buying this novel for me when I was in elementary school based on the title. My name is Angela, and when they saw a novel with a titular character sharing my name, they had to get it for me. Let me be honest, though; this is an extremely dark story for a children's book! Despite this, I absolutely loved it. I remembered so many things about it (especially the ending), even though I hadn't touched it in 20 years. When I came across it on Amazon for a cheap price, it brought back memories, so I ordered it and decided to read it again.
I won't give any spoilers away here, but there are so many moments in this book that feel like they're meant for an older audience, even though it's written in a way children can understand. The perfect, angelic Angela versus the evil, demonic Diabola is the leading point of the story, and the characters are so cartoonishly different that you can't help but keep on reading. Diabola especially gives the story that dark tone. Their toddler years have a goofy feeling to it, but once they start elementary school, things become more serious. As previously mentioned, the ending itself is so chilling that you will seriously question how many children read it without being affected in some way. The message the novel gives you by the end is a positive one for children who are trying to figure out who they are and who they want to become, though. I would give this book to my niece one day to read, though to be totally honest, I don't know what age would be appropriate for her to actually read it.
I personally love this story. I love the note of humor that's constantly appearing, the goofiness all of the characters seem to have about them (yes, even Diabola), and I love the dark and twisted ending with the overall positive message. Should children read it? Yes! When should they read it? Hmm...good question. My biggest suggestion would be to read this novel yourself first and then make the decision if your child is mature enough to handle it. I read it sometime between eight and ten years old the first time around and, even though I handled it just fine, it did leave some powerful memories behind. It's your choice!