This isn't an iPad problem. This is an idiot parent problem.
Laying that complaint at the iPad is like whining that you can't carry 4 people on the motorcycle you just purchased...
It has moved on in heaps of ways. Clients are far more powerful and capable of far more processing. 3d acceleration has become commodity. Compression, pixmap caching, etc. are now commonplace. Power consumption is a concern. Security is much more of a concern - bundling so much code into the X server, with the level of security access it has is a bad idea.
You just need to open your eyes and look at well... virtually any other GUI system from the last 10-15 years and see how most of them leave X11 for dead in terms of security, performance, etc.
The much vaunted "network transparency" of X11, the feature everyone whines that they will lose - is crap and done better by plenty of other software, from VNC to ICA to RDP...
Why is this considered acceptable? Get physical access to my iPhone (for example - Android is probably the same?), good luck getting in.
Sure, with a PC there's a few things that are a lot more difficult to secure (e.g., the boot process) but throwing hands up in the air and giving up because of physical access is a cop out.
"merely add a function to the X11 API" is the problem. X11 is ancient, full of bloat that no one uses any more and not designed with core concepts in mind that are desirable in a modern operating system. Really, look up some youtube presentations from the Wayland guys - who actually work on X11 and listen to what they have to say regarding the complexity and brain damage in X11. It works, but sometimes, even the guys who maintain it don't know exactly why.
The X11 display server is a liability and needs to die. It should have been taken out behind the shed and shot about a couple of decades ago. That doesn't mean that "oh noes i will lose my remoting!", that can be implemented in it's replacement via a shim, the same way any X display server works for Windows or Mac.