I wish I were a fairy would be a version that acknowledges subjunctive mood. It seems to be disappearing from the language, so that at this point in time both forms would be common enough to be acceptable.  I tend to vote for it in practice (I use it), but not insist on it too heavily. I have no idea when the subjunctive would be age appropriate.
     When I give the statement from my son to my classes, it's not to show that he is very advanced for his age, but to show what a somewhat typical five year old is capable of.  Our students need to learn a deep respect for the complexity of our unconscious grammar.  I gave it again here just as first hand evidence that children this age use subordinate clauses routinely.  I'm not an expert in language acquisition, but I do know that content clauses and adverbial subordinate clauses come very early.  When my adult grammar students analyze y son's sentence, they gain respect for the complexity of the clause relationships in a young child's natural speech. Students who are parents often bring in examples of their own. This is a priceless understanding, perhaps more important than anything else I teach.
     Whenever we teach grammar, we are somewhat forced into isolating language from all kinds of contexts.  I like the idea that you teach the parts as part of a whole.  Most ESL grammars seem naturally functional, since learning a language is learning to function within its community of users.  You also have a need to be systemic, since you can't get away with teaching the parts of the language that native speakers sometimes have trouble with, as many traditional grammars do.  Your students  need to learn the aspects of language that native speakers can and do take for granted.  I suspect that's why you have gravitated toward a comprehensive approach.  I'm glad that it's working.