• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
Yes that's obvious. County & duchy first. Actually just a county works too.

Depends if you have any of the kingdom's counties in your personal demesne. Click 'include lower titles'.

I wouldn't hand out the kingdoms at all, especially not to my sons.
 
I like to destroy extra kingdom titles, and grant all my sons a single barony that I build for them under my own counties. Then my heir gets my single kingdom title, or the kingdom and the empire plus all my counties.
 
If you cant hand a de jure kingdom to a son but to other persons, then you 100% have one kingdom or empire as gavelkind (or a duchy if duchy is your highest title). Includes titles you own and titles you can inherit. Check again and again. As soon as one major title (one of many) is gavelkind, the system doesnt allow you to hand anything above count to sons.

Anything else can safely be considered a bug.