Unilateral training does indeed lead to a difference between the size and strength of the muscles on each side. However, most active individuals have discrepancies between their dominant and non-dominant sides. They are measurable but rarely noticeable. Also, the contralateral side receives around half of the strength training effect of the trained side as a consequence of neural development. Hypertrophy appears not to occur on the untrained side.
Activities such as scooting, skating, snowboarding, and surfing inherently encourage us to bias our dominant sides. And the same is true of all racquet and throwing sports. It is worth noting, though, that the non-dominant side often receives a distinct but comparable training effect, too. On a scooter, for example, one leg supports and balances the body while the other drives it. Both are working throughout.
There are valid reasons to train both sides of the body: improved multilateral neurological and brain development, improved multilateral coordination, or for specific skill acquisition. (Raphael Nadal, for example, learnt to play tennis left-handed in order to have an advantage over his opponents who are mostly right-handed. He is naturally right-handed.) And of course for competitive bodybuilding, even subtle imbalances in muscle hypertrophy are disadvantageous.
For your daughter, it should be no cause for concern.