Is this a good idea? Young Adult Dungeon Dragon Playable?

[arbitrary librarian]

This all started when I set up a quest hook via some rumors of a massive treasure hoard hidden outside of the town the PC's were in. They took the bait and I had the final encounter be a dungeon dragon with his personal guard of dwarf fighter/engineer mercenaries. So after that was all said and done one of the players looked up a dungeon dragon and found the part that mentions them sometimes traveling with adventurers to get ideas for making their own dungeons.
So he wants to make a Dungeon Dragon character that is constantly disguised as a halfling that is traveling with the group and then reveal himself later on at an appropriate moment.
Any ideas on how to make this happen, or even if I should let it happen? I had thought about it for a bit when i stumbled upon dungeon dragons in the bestiary but I was more focused on getting the next encounter ready so i haven't made any real progress and between getting ready for college and setting up all the looting thats gonna happen next time we meet I don't have the time to mess with it myself.
Any and all ideas are welcome!

[Bandw2]

you could always do taninim, using this class. with the at-will humanoid form, draconic gift.

[EpicFail]

It's all about player balance ultimately with having a 'monster' as a PC. how would that compare to the other players' power level and how would they feel about it?

[Onyxlion]

Bandw2 wrote:

you could always do taninim, using this class. with the at-will humanoid form, draconic gift.

I vote for this the dragons or taninim as they call themselves are balanced nicely. I picked up the book myself and it's a hood value versus the few bucks I paid for it.