Similar experience here. Up until my DD was about age 7, she was "knock-kneed", meaning when her knees were together, her ankles were noticeably separated. It was very pronounced at age 2-6, then age 7-8 mostly resolved. At 11, now her knees and ankles touch properly when standing straight.
In addition, she has prominent knee caps (on long skinny legs!) - "knobby kneed" - which made her legs look bent even when she was squeezing as hard as she could.
Now at age 11, though, her legs (thighs in particular) are starting to "fill out" and the knobby-knees are much less visually prominent. Her legs appear straighter even though they aren't any different. I predict in a few more years, her larger thigh musculature will hide the knee-caps such that she has a lovely line, though not in any way hyper-extended like many more flexible gymnasts possess.
I have heard on here that stretching the back of the knee is very dangerous, perhaps not even possible without risk of injury. But the appearance of straightness, at least for my DD, has definitely improved over time just as she has matured.