To young for a combination lock?

My 9 year old is in 4th grade. She keeps telling us that things have gone missing from her locker. My boyfriend called the school and they said locks for the locker are not mandatory but kids are welcome to bring one. So we went out and bought her just a basic combination lock. Same as the one that my boyfriend, I and I’m sure all of you used in middle school and high school. My boyfriend and I have repeatedly shown her how to work it, and she has been practicing all night. She can not work it though. Like literally can not figure it out. Is 9 to young for a combination lock?