I would have handed the bag to him/placed on his lap with a simple but firm “your problem, not mine”
However I know that in the heat of the moment, sometimes you just want to minimise confrontation. OP, console yourself by indulging in a few moments of imagining what you could have done before ultimately shrugging it off and moving on, knowing you were the better person and some people are irredeemable boors.
A few ideas to get the ball rolling:
- Inform the crew there is an unattended bag under your seat. Suggesting, without explicitly stating, a security concern...”Does it need to be removed for destruction?!”
- Once in cruise with seatbelt signs off, discreetly lift the bag on your way to the rear lav and place in the locker above row 30.
- Loudly exclaim “Hey, cool, BA have installed footrests on these seats” while shuffling your feet & legs against his seat in an exaggerated way.
- Once in cruise, pull the laptop out and switch it on. Then tap 1C on the shoulder and say “Psst...is your IFE working? Mine seems to be asking for a password...do you know what it is?”
- Since you had kids with you...Ensure 1C can overheat ”No, you can’t go to the toilet now, the seatbelt signs are still on....you really can’t hold it?....OK, look, just go in this bag I found under the seats” while reaching for the bag.