GetChildren returns a table of tables because frames are tables.
Lua Code:
local parent = CreateFrame("frame","Foo")
local child1 = CreateFrame("frame","Bar1",parent) -- can also use Foo instead of parent, cannot be a string
local child2 = CreateFrame("frame","Bar2",parent)
local children = parent:GetChildren()
print( children[1] == child1 ) -- true
print( children[2] == child2 ) -- true
print( children[1] == Bar1 ) -- true
print( children[2] == Bar2 ) -- true
local count = 0
for k in ipairs(children) do
count = count + 1
end