Brian's right, at about 3 weeks old they jump up and hang upside down and chew through the bars, they are very good at this and after a week or so they can reach the bars just fine without any help.
Before 3 weeks old they should mostly be on momma's milk and shouldn't need to be eating block food full time.
In fact I don't let them eat much if any block food if I'm going to feed them before they hit 4 weeks old, only what they scrounge that the mom didn't eat.