Get a relative (grandma, grandad ?) or a nanny to fly with you. Get both of them Y tickets and let it be Y pax problem. You and Mrs. can relax in F.

Failing that, wouldn't she be tired by 8pm anyways ? I would have thought she would sleep. Take her to play area or splash pool around 3pm ? Though for some splash pool and flying may not be a good fit.

Load up tablet with her fav shows. I just download tons of Hey Duggie, Bing, Pablo and so forth for the kids. Between sleep and tablet and some of their fav treats it keeps them occupied. I've done 9 hours flights with 2 kids (both under 3) using this strategy and its worked. Tend to prefer evening/night flights since they tend get a good 5-6 hour sleep.