FWIW - Four seats in the Green Car cost roughly the same as three seats in Gran Class. It may be a good option when you want a seat for each child. If you reserve two rows of two seats, you can turn one row to face the other. (Also possible in Gran, but costs more, obvs).

TBH, For a three hour trip, I think I'd be very happy with a group of four seats in ordinary class, but YMMV.