First - your trees should probably be allowed to grow unimpeded for another year or two before considering your plan of action. The plants need foliage mass to thicken trunks, which is just starting to occur in earnest now.

In mid to late June (the summer solstice) your plants were in the process of changing energy sinks from foliage growth and branch extension to the cambial tissues, where they will be storing carbohydrates for their winter's rest and the spring growth push. They are laying down layers of cells, rich in carbohydrates, and thickening up now. To remove any substantial foliage mass now would seriously impact their ability to make food. You should wait until after leaves fall to do any major pruning.

Al

Your question seems to be a fairly common one amongst bonsai beginners. One can not overemphasize what Al has told you in his post; let it grow to develop a thick trunk before you do any pruning, training, etc.

Small, thin-trunked bonsai do not become thick-trunked bonsai. For the most part, bonsai start as thick-trunked, moderately large trees to begin with; only then are they reduced in stature and adapted to a small container.

Said another way, check out this excerpt from the article Field Growing Trees for Bonsai:

"A common misconception amongst newcomers to the art of bonsai is that trees (bonsa) with large, thick trunks must have had decades of training to become the size they are and that a thin-trunked seedling will one day acquire a thick mature trunk even though it is planted in a bonsai pot.

Unfortunately, once a tree is growing in the confines of a small pot, with its roots restricted and upper growth regularly pruned, the trunk and branches of the tree will only thicken very slowly.

Large bonsai with thick trunks are nearly always developed in the ground prior to being planted into a pot; some are purposely field grown, some are collected mature trees."

Have fun! Cheers!

Having it put that way makes excellent sense. I will continue to leave them be and reconsider the situation next spring. At that point it will probably be waiting another year. Thanks for the insite.