It's a green stick fracture:
"Definition: Because a child's bones are much more pliable than adult bone, an incomplete, or 'greenstick,' fracture may occur. A "greenstick fracture" means that one side of the fracture has broken and one side is bent; therefore it is classified as an incomplete break.
The name for a greenstick fracture comes from the analogy of breaking a young, fresh tree branch. The broken branch snaps on one side (the outer side of the bend), while the inner side is bent, and still in continuity. Most often the greenstick fracture must be bent back into the proper position (called a "reduction") and then casted for about six weeks. Greenstick fractures can take a long time to heal because they tend to occur in the middle, slower growing parts of bone."
It seems as though the incident occurred at kindy yesterday. Nothing was mentioned by LGR or kindy when we picked him up and it wasn't until almost bedtime last night he started mentioning his sore arm. We gave him some Nurofen and he slept pretty well.Over the course of today the arm has gotten bigger and bigger and he has used it less and less. The GP took a look at it and then it was off for X-rays and then the bad news. As I sit type LGR and Mrs G are somewhere getting a cast put on while Hayzey patiently waits for dinner to finish cooking.
Should make life interesting for a while.