Greetings Brenda:
If I'm reading this correctly, your young black walnut trees are sending up new basal sprouts from the root collar of the tree...correct? Moreover, you stated that the main stem is not dead...correct?
If the main stem is not dead, then I would allow the basal sprouts to grow a little more to determine if the basal sprouts display better form and vigor than the original main stem. If the sprouts are straighter and more vigorous than the main stem, the simply cut the main stem back to release the best basal sprout.
This recommendation is based on the premise that most owners want straight, singled-stemmed, and well-formed trees.