but I say, why waste money on a big seat in First for someone 4' tall?)
Can't speak to trains, but there's no way I'd do long haul with kids anywhere further back than CW (or futher forward - flew BA First Singapore to Sydney, was not enjoyable, the seats aren't built for families)
Ultimately though you aren't booking the seat for the kids, you're booking it for the entire group, and having more space is always a good thing.