How to order nodes in Tikz mindmap

Add [main/.style={sibling angle=180}] to your tikzpicture like this:

\begin{tikzpicture}[main/.style={sibling angle=180}]

Then you can write the part with the node 1 like:

...

child[concept color=blue] {
    node[concept] {1}
        [clockwise from =0]
    child[main] { node[concept] {\scriptsize{a}} }
    child[main] { node[concept] {\scriptsize{b} }}
};

...

And here's the result: