1 string, use a candle or match to burn the edges for easier stringing or wrap tape around the edges.
several beads with large holes for easier stringing.
because this is a toddler craft, you don't want to worry about connector pieces on the end of the string, just tie it up and make it long enough that the child can pull the necklace over his/her head. big beads are also key. you want the child to be able to string the beads his/herself. natural wood beads are easy to find at your local craft store and perfect for this type of thing.