At first I was going to say that they are redundant, (and there is some overlap), but upon paging through I think the books fit different needs. All Year Round -as you know- has as it's focus the seasons/festivals and handwork that relates to the festival or for the nature table.
I think Children's year is really full of handwork for children (lovely things that we can make to surround our children with) or handwork that can be made by 1-2grade children and up---several different doll patterns (instructions for doll heads), different patterns for knitted animals, directions for sewn/knitted clothes, hats, slippers, etc.
We have made many different gifts from Children's Year; at least that is how it has worked out for us--when my dd wants to find something new to work on, TCY pretty dependably has something that she'd like to make--and at this age we work on it together. I used AYR a lot more when I was building up our nature table characters, and planning out how to celebrate different festivals.
If you can I'd get both, I think they're worth it.