I am guessing that you are not a parent because you are playing favorites. Don't you want to develop the other 3 to their full potential ?
You need to allow everyone to develop and give the other girls (who I am guessing are paying the same rates for your business) their own opportunities to advance. Kids are very different some will shine in some areas others need different opportunities but every kid needs to be provided an opportunity to do well. If they fail, you need to explain that to them as well. Good luck kids are not for the faint of heart.