Do Catholics truly believe that an unchristened child goes to hell?
Children who have not reached an age of understanding are in no danger of going to hell.
Jesus himself said in Mark 10:13, "Let the little children come to me, and do not hinder them, for the kingdom of God belongs to such as these. I tell you the truth, anyone who will not receive the kingdom of God like a little child will never enter it."
It says nothing about being baptised, christened, or anything of the sort for a child to enter the kingdom of God.
Before a child has a sense of what's right and wrong I believe they are pre-saved.
Once one acquires a sense of right and wrong then they must make the choice to accept or reject Jesus in order to be saved.
They go to limbo.
EDIT: that's what the Church says. I don't give a shit.