We're doing that same thing this Christmas. One three year old and one five year old. Should be a good trip.
Unfortunately, we're not eating at Phantom. When I called they said that the price is per person and the 3 and 5 year old are the same price and no meal sharing allowed. It'll be freeze dried Christmas Dinner for us!
That's a shame about the price. Our son was not yet two and no one said a word about him sharing a plate. I would really like to here how it goes with two kids. We had our hands full with one. The canteen is still a great place to hang out before and after dinner for hot chocolate and there is a small collection of games and books there too.