We took our (just) two year old girls across the pond in October in old CW. I'd keep an eye on the middle 3 seats on the upper deck on the A380. If there's a spare row, this will make your life much easier. Our three were pretty good, but by that stage definitely needed their own seats. Then again, us adults were outnumbered so I find it hard to judge. Maybe also monitor Avios availability for after her brithday for the return, too? CWS is unlikely to work well with her in her own seat, but old CW is great.

I'm sure you'll have a great trip.