Running is leaping. Land with knees bent and feel the spring forward.
Your son's motion sounds like a good sprint motion.
As your daughter gets stronger, she will be able to take longer strides because she will jump forward rather than 'churn'.
Mechanics:
If you bound, the hip rotation slows.
When you can leap far forward, and only slightly upward (very acute angle with respect to the ground), that is the motion.
So...jump as far forward with each stride without losing momentum.
Herman Frazier, Tony Darden, James Gilkes, Jerry Bright, Tommie Smith, Matt Boling---examples of long strides.
Percy Cerutty:
Learn to run with hips on feet, rather than with feet on hips
Turn your hips at max speed in their full rotation while jumping as far forward with each stride as possible without slowing hip rotation.
Sounds more complicated than it is.
Trying to speed up implies being able to jump another inch or two with each stride without slowing hip rotation. If it is not there that race, training for a few weeks might get that extra inch or two.
What most runners do:
Shortening the hip rotation to get their feet on the ground more is what most sprinters do if in a 400m, and almost middle-distance or distance runners do this no matter what the race.