Lisa, are you asking for idea's for designs or want links to the designs.
here in free designs they have great large flower designs. I embroidered them on my shirt but on jeans will look great too. Look at dress-elements, and flowers.You have to sign in, but they are for free and stitch out very well.
What I have done in the past is embroidery FSL designs then sew them up the side of pants legs. H&*.
Lilian gave you great advice! Also, take a look at n-needlework.ru, they have a lot of free designs called 'dress elements'. These are flowers, sprays, things like that. A lot are one color work, which could be dramatic on jeans. Good luck, hugs, Marji
If u r a paid member of cute, there are untold numbers of designs on this site that would be great for the leg of girls jeans. the age of the girl would come into play & u might want to add some glitz & glamor with sequins or bling. .Animals, purses, shoes, whatever this young lady likes. *