Does she have thin silky hair?  I've never found a barrette that was tight enoughor small enough.  Sometimes the snap style will work but eventually they wear out.  I like the claw type.  Like this:
 http://tinkerbellscloset.com/items/b...lip-detail.htm 
They always hold Lola's hair and you can use them for a top knot or pigtails.  Plus they don't rip out her hair when I take them out. I just ordered some baby snap styles ones to try out for bed time.  I'll let you know if they work.