My yorkies are great climbers so I keep them in crates. However, if you want a quick solution, try covering the pen with a bed sheet. If you are afraid the bed sheet will fall off, buy some clips and clip the sheet onto the pen.
p.s. here is a picture of one of my dog escaping the bathroom when she was around 6-8 weeks old.
http://www.geocities.com/tinybittosalt/MVC-004V.MPG