Personally, i say tell tehm before you go, (I have to anyway, my kids do NOT deal well with surprises -(autism), but the excitement of counting down sleeps, packing bags etc makes your holiday feels like it begins earlier. We make a little count down calendar with matchboxes or something similar which we fill with little stickers or sweets for the last few days leading up to it. Our last trip was sep, and our next will be about the same time, and we haven't had a day yet when the kids don't ask. how many sleeps is it now? or talk about their favourite rides. It's nice to know that when i'm scrimping and saving to pay for it, they're really appreiating it1