Play ball with them, but don't coach them. They are your kid, not your student. All you really have in life is time, so how you spend that time with your child is the best, or worst thing you can do with your life.
If your softball sessions are stressful to either you or your child, then you're doing it wrong. You should be having fun, they should be having fun. Challenge them, but the last thing they need is another critic telling them what to do and how to do it.
Hit them ground balls, play catch with them, help them practice. Ask them to ask their coach to give them "homework". Have fun.
-W
Agree - I have to remember this because I naturally want to put the pedal to the metal. Your DD instinctively wants to please you as a parent and the coach because they see both as authority figures. I also would suggest to keep it light and fun so your child doesn't feel the sport is work rather than a game. The pressure can build even if we think we are just trying to be helpful.