The big problem with source filters is that they can break easily. As long as you are using for introductory purposes, though, I think such a filter would be fine.
Your best bet if you want to try this is to code up an attempt at a Spanish filter. Install Filter::Simple, make a list of keywords, use a dictionary to come up with a start on the replacements, then talk to an interested native speaker to fine tune your list, then throw something together.
Take a look at the source of Acme::Lingua::Pirate::Perl&emdash;Filter::Simple does the hard work for you, all you have to do is provide the transformations. The start I provided above should only need more transformations added to make it useful. Not that I consider what I did as significant, but please consider it public domain and use it as you will.
If you are thinking about working with younger kids, you might want to look at using Scratch in Spanish.