Yeah, all my spellcaster + Spellcaster pairings have resulted in Spellcaster kids. I only sometimes get non-spellcaster kids if one of the parents is not a spellcaster. There have been a few non-spellcaster kids in otherwise spellcaster homes via MCCC, but I always figured they were from previous/other spellcaster+non-spellcaster relationships.
Now, there was one sim, not one I was playing but one who was via MCCC's family progression function who was a non-spellcaster but otherwise had all the traits (strong bloodline) and all the rest of the household were spellcasters -although his father wasn't part of the household. (MCCC can make for some tangled family trees) I used MCCC to make him a spellcaster before he married into my main Sim family.
Weirdly, despite him having Strong bloodline and my sim family having ancient bloodline, their kid didn't have any bloodline trait. I figured it was a glitch due to mods and cheated it on.