That's right, LRoM sure does like his duct tape - as does, I hear, LSoM. Oh yes. But it's not for us mere commoners to muse on the goings-on behind the sturdy portals of Mannion Towers...
baldbrit - hello. Thanks for the PM, I don't get many and it's lovely to feel wanted. Thought it might be best if I relpied in this thread so that it's there for reference for any other mug who's thinking about travelling with a sprog.
It's true that the UC seatbelt is way too big for your average 2 year old. Thankfully, those clever boffins at VS have developed an ingenious piece of kit to help improve matters:

Yes. It's a pillow. If you've skimmed through my last TR you'll be aware that jr. discovered that standing up was an awfully good way of getting out of her seatbelt however, and different measures need to be employed to try and avoid this. M&Ms worked well for us and you can never have enough toys, books, stickers etc. to try and keep them distracted.
And as a parent yourself you'll already know this, but don't underestimate the power of bribery and threats. If staying sat down until the seatbelt sign goes off can result in a reward or - conversely - a good beating, then jr. will be much more motivated to keep his/her arse firmly planted on that coveted purple leather. OK, maybe not a beating. Just tell him Santa won't come next Christmas, that tends to work...
Hope that goes some small way to putting your mind at rest. You'll be fine, stress not, I'm sure he's a good kid and they do tend to rise to the occasion. Enjoy your flight.