You may already know all of what I am going to tell you, but it is information that I wish I would have known going into all of this.
As a parent that started kids really young - I just have a few suggestions. If you get new skis they are usually over buoyant. These little ski's will frustrate kiddos. Skis will get crossed and have a really hard time getting up. HO and probably others sell a combo ski tail weight that keeps the tips naturally up out of the water. Until you have a chance to get tension on the line kids normally have the most trouble and many times they want to be done before the whole fun starts. My other suggestion is to look for wooden, less buoyant skis and if your kids are less then 4, I would stick to a skimmer ski trainer.
As far as talking to your kids, I would give them 3 rules to skiing success: 1. knees bent 2. handle low. 3. elbows straight.
Most importantly, make sure they are having fun. Some kids need a little tuff love, but if anything helped my kids, even more than instruction, it was all the cheering and high fives for accomplishments they had.