We are moving away from age-specific labeling since age is not a great indicator of skill. We have even seen many 4 YO doing our advanced lessons and some 9 YO doing our beginner lessons. There is nothing wrong with this. Instead, we are in the process of categorizing our lessons into difficulty levels (i.e., preschool, beginner, advanced, intermediate). We are not quite finished, hopefully soon, though.
I would recommend simply asking your children what they would like to draw and encourage them to try even lessons that may look challenging. Each lesson is broken down into much smaller steps; what may seem challenging is very doable.
Also, remind your little artists that it's ok if their drawing doesn't look exactly like ours. The most important thing is to have fun and practice :)