If they are on the same path they can disclose their discoveries with one another. i would expect that your daughters would be paced closer together than your son unless they are exceptionally bright or he is exceptionally dull =). Eventually, hopefully the girls will go to your son for information and he'll be able to teach some things to them. I find that nothing solidifies an understanding of something than teaching it to someone else. This would be mutually beneficial to all of them.
The only thing i really took issue with that i read in the comments above was:
...I'd say PHP and SQL, JavaScript seems harder and a little bit useless to learn.
SQL is useful, but you don't write much on your own with SQL (I believe that by the standard it isn't turing-complete, though some vendors make their implementation turing-complete via extensions), it needs to be coupled with another language and likely a library to access your RDBMS.
However, I simply have to say that PHP should be avoided this early on. I know it's a tool that's out there in use, but in terms of "academic" use, i think it's poor. Certainly don't start with it. There is no formal specification for PHP, and it just feels sloppy to me. This is just my opinion, but i'd really try to avoid PHP at this early stage.
The other point that i took exception to was the bit about JavaScript. JavaScript has a lot of great features (It's Object-Oriented, functions are objects, etc.). I'd say it's really hamstrung by being thought of as a "web thing", and having the primary interpreter for most people being in a browser. If your son is already rolling with HTML, JavaScript would be a way for him to start actually programming while leveraging the presentation layer he's already learned.
I definitely don't think 11 is too young, and 13 certainly isn't. Their brains are still spongy, and they are probably already very familiar with using a computer (certainly more so than I was at that age).
It's great that your kids are interested and that you're involved in helping them learn. I think just getting them going on anything and encouraging them is going to go far, no matter what option you choose.
-Lee