The Polar set offers the right amount of a challenge for your youngsters. Jugs are known to be the easiest kind of hold to grab. This set is more like big edges where little fingers can get back into the hold. A jug on a vertical wall is considered V0 in grading, and the Polar Set is rated at V1.
They all have a rounded lip just like a cobblestone, so they do get enough of a hold to succeed but not so much that they can jump around like a monkey; because we all know monkeys don't live where polar animals do!