First guess, and I think this fits well with what Jim originally wrote -- the IC father / Man Upstairs (Will Ferrell) would have a drive of Certainty with his wanting to glue everything in place. He Changes to Potentiality when he sees the potential for playing lego with his kid.
Playing and building Lego is definitely doing, so I'd peg Finn as a Do-er. Wanting to spend all his time playing out imaginative stories with Lego, and being unable to resist the Attraction of his dad's awesome collection, works well as a Concern of The Present. Finn's steadfast drive would be Proaction which fits the way he goes ahead and plays with the Lego against the rules, and tries to convince his dad to change his mind. (Note the MC Response of Acceptance there too.)
Checking MC & IC Domains for a minute ... IC Domain of Mind / Fixed Attitude definitely works, with dad's stuck opinion about his Lego models being only for looking at. MC Domain took a bit of thought, but I think "being a boy in a house full of unplayable Lego" or "being the son of a Lego geek" work pretty well. (Part of his situation is being a kid without any say.)
The OS is pretty much the same as with the main story (just with Symptom & Response flipped). In the substory, the Goal would be something like, conceiving a better way to use all this Lego. The Issue of Permission fits really well in the substory too -- not being allowed to play with all this great Lego.