To make crib: Using serrated knife, cut plastic dowel rods--16 pieces 8 in. long, 2 pieces 10 in. long, 4 pieces 7 1/4 in. long. Attach 8 in. rods to sides of cake with buttercream, approximately 2 1/4 in. apart. Glue two 7 1/4 in. rods together for each long side of top railing; attach 10 inch rods for short sides of top railings. Roll fondant into rope and wrap around corners and middle of side railings. Using craft knife, cut holes in one end of to rail to line up with vertical railing for mobile.