If you're looking for luxury, definately Vistana. You didn't mention if you are looking at the Resort or Villas? We've stayed at HIFS twice and VV once and loved them both. Your one and five year old will definately prefer HIFS, however. There is much more for them to do there, including an arcade, pool toys, playground, putt putt, train rides, clowns, face painting, and poolside games.
Try and do a search and you'll find many old threads comparing the two. You'll be happy with whichever one you pick!