If you have established your kingdom your grandchild has nothing to do but kill his wife/nephew/etc. Can't go for wealth/prestige/pious as the previous kings have sorted all that out for you. Maybe change it to a system that tracks the average within Christendom?
So say for prestige, you need what, either 500/1000, I can't remember. If you're a king you have have the ambition of being the most prestigious, most pious, most wealthy... As it is, you should inherit enough wealth and the rest comes very easily.
Also, ambitions to take new land? to take part in holy war? take titles off of other rulers?