Unlikely. This is not the type of thing random people come along and contribute to. Short answer is because the API doesn't allow sorting by the number of child items, which in turn, I'd assume (though that part I don't know) has to do with the way items and their attachments are stored on the server.
No offense, but there's no new information in your comment... except maybe that you guys are quick to discourage someone who might have otherwise spent her weekend looking into a new feature...
I mean - it tells you that the online library is based on the API, that the API's order attribute doesn't accept no. of children as an argument and that it's likely hard to implement. Given your comment I have no way of telling to what extent you have or haven't looked at this at all, so that seemed like useful preliminary information.
If that doesn't deter you, i.e. if writing server/api code in php is something you do, you'll probably want to have a first general look at the dataserver code on github and then ask specific questions over on zotero-dev https://groups.google.com/forum/?fromgroups=#!forum/zotero-dev and Dan would be the person you need to answer those.
Upgrade Storage
If that doesn't deter you, i.e. if writing server/api code in php is something you do, you'll probably want to have a first general look at the dataserver code on github and then ask specific questions over on zotero-dev https://groups.google.com/forum/?fromgroups=#!forum/zotero-dev and Dan would be the person you need to answer those.