I wouldn't recommend getting started with this first thing in the morning. Take advantage of short lines for attractions at the beginning of the day and start this later. I'd wait until you have a quiet time in your plans - and the line behind the Christmas shop is almost always shorter and faster. With no line - 10 minutes to get started. But - the 20 minute suggestion is probably about average.
They will ask you now what LAND you would like to start the game in. That's nice. So - when you start - pick the land you are going to be in next on your touring plan. You'll do several portals in one land before you move on to a different land.
For first-timers - you don't need a binder or anything - but I might bring an envelope with each kid's name on it to put each of your sets of cards in. They are trading card size - so just a small letter size will do. It helps keep the cards in good shape and not lose them in random places. And if they decide to collect them - this becomes more important later on.
NOTE - SOTMK players - especially ones with a binder of cards or are holding up more than one card to play - are experienced and "usually" friendly (and probably playing a med or hard level, not the easy intro level). If they offer to trade - make sure you trade "like card for like card." Mainly - know that there are "star" cards - there's a symbol on them - and those are numbered 1-22 - and those are harder to find - only one per pack. Don't give away a star card for a non-star card. Your kids might want a particular character more, but if they get into it, star cards are harder to get. Sometimes experienced folks will give away a card or two to a new player, too. (Don't ask - but if you strike up a conversation, they might offer.) I've got like 6 extras of a common card now - so for example, I might do this my next trip.
It's easiest if you all play the first game the first time through because different games could split up your party in different lands. If you have more than one kid - just have them trade off who battles at each portal.
Sometimes newbies just want to cast their spell and move on - and don't really listen to what the portal story is. That's okay! It's intended to be fun however you want it to be. BUT - you do need to make sure they stop and pay attention to what the next portal they are being sent to. Some little ones get ready to move on too soon!
There's a nice facebook page about this all, too.