My suggestion for figuring out a run up for any athlete to do find it away from the pit. For example, put the kid on the straight on the track and have him/her do their run up without a pit in front of them. Once you get their steps consistent, and I agree with mn_highjumper that consitency is the key, you can move them to the runway or pit.
Often times, especially young kids, will get nervous when they see a pit or bar approaching them. Getting the steps down away from the pit gets them used to being consistent with their steps during the approach.