It’s a guess in the beginning, but kids have a much wider range of prescription that works for them because their eyes can turn inward or outward to focus (accommodative esotropia or exotropia), unlike adults. You give a general prescription and see if it fixes the eye crossing or drift. If not, up the prescription.

They can also give kids eye drops that relax the eyes and then they use a machine to look at their eyes and get a more precise reading and prescription.