Judy Bennett ( who plays Shula ) used to be the voice of many of the children in The Archers in the 1960's and 70's.
I think it was just the mechanics of recording that adults would voice children in the early days. Some child characters are too young to read, and wouldn't be able to act on que. There are time limits on the hours child actors can work too.
The boy who plays Ruari has his lines read to him and he repeats them. He is recorded on his own and has no idea that his "mother" is dead.
Editing is so much easier these days, so the recording can be put together so much more quickly than would have been possible before. It is easier to use children to play children these days.
Judy Bennett voiced many puppet series in the 1970's like "The Adventures of Rupert Bear" and "Cloppa Castle" with her real-life husband Charles Collingwood ( Brian Aldridge in The Archers ).
http://www.toonhound.com/cloppacastle.htm