Or should they be considered "off limits"?  Does it matter if they are minors or not?

To the extent that they have a public life, yes.

Minor children of candidates probably don't have much of a public life - they don't make their own speeches or have a career in acting, for example - and there's really not much to tell beyond what the parents want to make public.

Grown children - Biden's son going to Iraq, or his other ex-lobbyist son - are another story, but it would take a lot for my opinion of a candidate to be swayed by most activities of a candidate's child.

The more attention paid to the candidates' children, the less paid to the candidates. Typical election year distraction and misdirection away from the issues.

Children are off limits.  However, all issues relating to the candidate's character are fair.  Accordingly, one may talk about parenting decisions.

They're off limits unless the candidate puts them in play.