Unfortunately the rude people already got to this thread. Bad inflight behaviour happens at any age, usually peaking around age 21 in my opinion! In some cases, such as a recent flight where the aisle of the CW cabin was obstructed by other passengers, at age 40. The CW aisle on an overnight place is not the place to stand holding your drinks and joking loudly like it's a City bar, Gentlemen!

Meanwhile, to discuss the actual question:

Bring your own favourite (quiet) entertainments, iPad, books, etc for her. IFE content can be limited and it can also break down.

Perhaps also bring favourite clothing, small pillow, toy, etc as she may like. If she is a fussy eater, bring some of her preferred (security-compatible) foods, or acquire them in the airport airside. It may be good to have some of her favourite snacks along in any case.

Perhaps try discussing how this will be a different flight from usual, show her pictures of the cabin, etc, so she can realise this will be a relaxed, spacious experience; discuss that the food will be more like a restaurant with several choices. In the lounge, find a convenient corner or sofa for both of you to hang out and remind her that it's someone else's house so you have to be quiet.

The CCR staff in LHR have been known to do children's versions of the cocktails so she can have a drink that looks like a martini or similar if you wish. She might like hanging out on the CCR terrace and looking out at the airport.

Inflight the boarding experience, particularly in F, will be quite un-stressful and if you get a window seat, the view out of the window is great.

I hope you have a great time, and try not to let the naysayers irritate you too much. You are as entitled to be on the aircraft in that cabin as anyone else who is ticketed there.