I had the same problem when I first started learning to play bass in grade school. Fingers not fully grown makes it harder than it is now but I'll always remember one trick my teacher had taught me, it may sound odd but try taking something like a VHS case (it's what was around at the time) and make lines about 3/4" apart. Keeping your thumb flat against the back work your fingers like they're on the guitar. You can sit there for hours watching TV working the muscle in your fingers. It may not work for you, but it sure helped me. I'm currently using this trick again after 7 years away from the electric.