Here is my own 2 cents. I have 2 boys now ages 9 & 6. They have both been skiing since the were 4. I read all of the posts above and agree with most of the remarks. My highlights are:
1. Alternate activites - tubing and swimming top the list.
2. Time expectation management - The younger ones will not have attention span for a full day or possible even a half day. Just be aware - not that I need to tell you about your own kids.
3. Cold - When my boys were very young they did not like being exposed to cold blowing winds. It ruined my older sons first day in ski school. Spring skiing is a great first choice. (Although I saw that you already booked your first trip.) Send them to ski school with a neck gaiter or something similar in case the winds blow that day. Which leads me to my next point.....
4. Ski school - I highly recommend it as do many people on this site. Neither one of my boys liked it at first because they were not with me, but the older one was able to competently ski blacks at the age of 7. (The little guys said last week that he does not want to go back to ski school.) It will be money well spent.....
5. That being said, if you are capable, any time you personally spend teaching your kids can pay off ten fold. At least it did for me. Now there are those around here who will vehemently disagree with this point, but I am a coach for 3 non-snow sports so it is in my blood. I spent the better part of 2 seasons on bunny slopes with my older son and I was able to work him through his concerns and point out his mistakes that were becoming repetitious. This is a very personal decision.
Lastly, make it all about fun and you will all have a blast. Take it slow and easy and don't push too hard. I wish you all of the best and i believe that they will be posting here one day in August just like the rest of us crazies.