Can a hermit be a name for someone who never has children?

No; that isn't a very good use of the Word. While it is true that some hermits do not have children, people who do not have children are not necessarily hermits. There are true hermits, (eremites, monks who live eremitic lives) and there are eccentric or agoraphobic people who you might call hermits. The term is also broadly used for some people who are not really hermits but who tend to be loners rather than sociable.