You can still educate your grandson through diplomacy generally. Just choose educate child under diplomacy with your son.It all depends how your monarch lives, with good traits you can have as many lands as you want your vassals will love you anyway... So be brave, Pius and avoid being asshole... but be Dick to your Heathen neighbors: Which make CK2 such great RPG game. Only drawback is that such successful Monarch tend to live 80+ so your heir will be raised by your son and turn to be total fop and sodomite... But this how history of Jewrope rolled otherwise some Dynasty would color this map by ages ago.
Sometimes when I tried it they refused due to risks involved or something like that (would become a hostage?) which doesnt really make sense but the point is you cant count on this. Maybe they patched it out.