measuring elastic for bloomers

I made a child's size 4 dress and bloomer to give away.
The directions say to measure the child's waist and leg for the elastic length.
But I don't know the child.
The pattern does give a standard waist measure: 21", but what measure should I use for the leg/thigh

My little sister is almost 5 and her thigh/leg measurement is 10" if that helps.

Thank you.  That is the leg measurememt I wil use.

Try to use 10-12 measurement, our mother used 11 for my 5 yr old little brother. He likes it because he is comfortable.