You are absolutely right about fluency being a skill that builds on practice. There is no secret there. In regards to children with special needs, latest research have actually shown that teaching a second/foreign language to a child with linguistic related issues help that child getting stronger in the first language.
Before such research showed the benefits of bilingualism/multilingualism in linguistically-challenged children (motor, neurologically, auditory, etc.), it was believed that literacy in the first language was necessary before a 2nd language could be introduced. Now, the neurological exercise that your child is getting by learning other languages is strenghtening the synaptic connections relevant to linguistic learning and production. And since several different parts of the brain are involved in the skill of language, learning multiple languages forces the brain to re-wire in order to make sense of all this new stimuli (plus long term potentiation and memory processes).
If you have time, you should read Joseph Ledoux "Synaptic Self", or maybe an easier read would be James Zull "The Art of Changing the Brain: Enriching Teaching by Exploring the Biology of Learning." Zull does not address bilingualism specifically, but some implications are evident. And you having been a teacher, will definitely see them.