M is just at 4 ft (she was so excited about that because she got to ride the roller coasters at Universal lol) but is a skinny butt, she definitely doesn't get that from me! She has to have 7/8 pants for the length but is so little around that they have to be super skinnys and have adjustable waists, it's frustrating. She's a little muscle, she plays several sports (soccer, swim team & gymnastics) and rides, I wish I had 6 pack abs like she does.
There was no getting her to be assertive. It's a chore to keep her bravery contained. She's been that way since she started walking at 9 months old. The wanting to run barrels is the least of it, she also thinks she needs to be jumping Missy. Nope, no way, not happening.
Don't worry yet. At their age, they are usually one extreme or the other. I always tell parents at first lessons that most kids cry the first time they get on a horse, depending on the kid it's either when you put them on or when you take them off. You might try finding things to make it fun & give her a goal at the same time. If she is comfortable on the lunge line (and with a horse that is cool with some odd things), play the "mummy game" - give her a roll of tp, you hold the end, her challenge is to make nice circles around you and wrap you up.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.