We have 3 kids doing catechism... their ages are 7, 5, and 3. My oldest knows about 70 or so, the middle knows about 45 and the youngest is a young 3 year old, just starting, and he knows about 10.
Our nightly family devotions are LONG... about 45 minutes of singing, prayer, Bible reading... and then we get to catechism. Going through each question would simply be too much. So... I usually pick one question per page just to keep things fresh in their mind... plus the act of giving them the question out of order prevents them from slipping into "auto pilot" mode. On Fridays we have a "challenge" game where I ask a question and the first one to raise his/her hand gets to answer and at the end of the game the one with the most points gets a special daddy tickle/munch session. (Conversly, the one with the worst score gets a severe beating.)
(Just kidding about that last sentence!)