If she'll be middle of the pack on the U10 team move her up and don't look back. Ask a few other other coaches in the league to give you their opinion of how she'll do so you get an unbiased opinion of her skills.
If she's going to be the 12th best girl on a team of 12 for your U10s, let her be a big fish at 8U and let her practice with your U10 team.
If she's going to be the 12th best girl on a team of 12 for your U10s, let her be a big fish at 8U and let her practice with your U10 team.