The answer of why they haven't increased them is simple. Email attachments cost money (bandwidth) of all the intermediary nodes. Email is not a point-to-point service, it is store and forward. So, that all the systems along the way, need to store your large attachment. Now, upload services have the same issue (and also have limits, just larger ones), but at least it is known that there is only one recipient. Anyway, I don't know any service that allows you to upload a petabyte file as an ordinary user. Maybe Netflix can do so, but they have special content delivery services that try to move data close to its targets and use "streaming" to carry large files. You don't get your movie as one chunk, but as a streaming of smaller ones that are buffered. So, there will always be limits and they are much smaller than you imagine. And there are technologies to mitigate (not remove) those limits for specific use cases. But, unlimited sized email attachments is a pipe dream and will never happen. If they did, it would simply cause people to want to send bigger files. This is the "crisis of the commons" played out in file sizes.

I discuss internet safety on twitter for a living and family related internet safety because one can't help but get involved. I don't have any specific recommendation for blocking software as I don't use such myself. My child just uses the internet under our supervision and hasn't run into any porn yet. However, what I think you really want for your child is a place where they can safely go and will learn to be a responsible net citizen. For that I recommend checking out Yoursphere. It is a social site designed to be safe for childern. The link in the subject is their intro for parents. I'd also check out "ilookbothways.com" as a place to start educating yourself.