Adding labels and fields to a nested JSON
The rgp loop can be made more compact, and a bit faster, with:

def foo2(rgp):
    alist = []
    for n, g in rgp:
        temp2 = {"name": n}
        values = g.T.to_dict().values()
        n = len(values)
        def size(t): 
            return (t['Rating'] * t['Count'] * 10000) / n
        temp3 = [{'name': t['Name'], 'size': size(t)} for t in values]
        temp2['children'] = temp3
        alist.append(temp2)
    return alist


I don't have enough experience with Pandas to know whether it is possible to improve on the groupby.  For example, it would be possible to perform a 2 level grouping with one call - ie. group on 'Count' and within that 'Rating'?

Considering that json is a string version of a dict, and you have a specific dictionary layout in mind, I don't see how you can organize the code in any other way.  With the exception of update, all dictionary additions are key by key.  So you have to have these 2 loops over groups.