My crank slips I think
My son and I have had direct experience with this (he is helping me write this).  I realize this thread is very old, but I put this here in case others are finding it.
First, check the chain.  If it is worn too much (they elongate over time) it won't grab the teeth.
Second, Check the chainring,  Are the teeth sharp?  If so, you need to replace it?
Third, check the teeth on the rear cog.  Again, if the are sharp, you need to replace it.