Your choices should really be made on where you are skiing, how early or late on in the season as well as how much skiing you are planning to do. For example, younger children just starting out in ski school are often low down in resort where it is warmer but have a lot of standing around waiting during ski school so it can still get cold.
Older and more experienced children often are working harder so generate heat quicker and will need more “active” layers to ensure heat generated wicks away and does not wet the clothing which is when the chills set in on ski chair lifts.
Ideally when skiing, we would recommend always wearing a base layer or skiing thermal whatever the weather. This is probably the most important layer as being next to the skin its job is to wick away any moisture away from the skin and dry quickly. The differences in pricing found in skiing thermals are down to the material used generally.