These are seedling trees growing in the ground? If so, they do not necessarily have to have been planted intentionally......Japanese maples can seed rather freely by themselves if conditions are good. The seedlings will be just generic J. maples as named forms are virtually always grafted. They may also be quite different in appearance from the parent tree(s), often triggering the assumption that they are distinct cultivars (which they are not). Any seedling J. maple allowed to grow to maturity is simply an Acer palmatum, not any specific cultivar. The cultivar name can only be applied to grafted trees or those grown from cuttings (rather difficult to accomplish).

If I understand your question correctly, you are asking if you can keep these seedling trees small just by pruning. The answer is you can keep them smaller by judicious pruning/training but not easily dwarfed to a few feet unless they are by intent already dwarf forms of JM (legitimate grafted cultivars). Pruning to keep small is NOT the same as bonsai. Bonsai is a complex process that involves limiting root growth (by careful root pruning and limiting container size) as well as by training and selective pruning. It really does not translate directly or even well to inground plantings.

Even though J. maples grow rather slowly in comparison to other tree species, a generic J. maples that wants to be 15-20' tall or larger will not respond well to pruning to keep it to only a few feet. You will not be happy with the appearance and the health of the tree will be compromised. If you truly want to keep these seedlings small, research bonsai or take a few classes and grow them as traditional bonsai specimens. Or transplant into containers, which will tend to act as a natural dwarfing process, although you will need to follow similar care practices to traditional bonsai (frequent root pruning, replanting, careful training and selective pruning). It may be easiest to simply transplant the seedlings to locations where they can be allowed to grow unimpeded.