Other reviewers have suggested they added the hole-in-the-wall laser and RC car only to add lasers & complications to the plot. But the reason is obvious to me: I'm sure there was a radiation detector at the front door, and he'd sound the biggest alarm in the place if he tried to carry it out the front.

That said, the kid is remarkably lucky to have his plan(s) work perfectly on the first try, without much access to make the plans.

This is a fun movie, just humorous enough and very smart, and if you always wished you were a nuclear physicist as a kid, you will enjoy it!