I have not had to explain the reasoning behind the treatments to my son who receives them, but I have to his older brother, who is 6. I tell him we treat the birthmark to keep it from being "uncomfortable". Uncomfortable covers turning purple, having blebs, social awkwardness, etc. without having to go into it. That is more information than either of them need yo know at this age. The very simple answer is all my 6 year old needs at this point and may be all your 5 year old needs too.
Hope this helps!