Theoretical physics, at its heart, is all about experimental physics. Let me illustrate this statement using the concept of energy. First of all, there is nothing mysterious or vague about this quantity. Energy is defined as the ability to do work. Work, in turn, is defined as the force applied to an object multiplied by the distance the object has moved during the application of this force. Hence, energy is the ability to sustain some force over some distance. Knowing this, how would one determine an exact theoretical expression for the energy density of light? Experimentally, it is simple enough and involves letting a beam of light be absorbed by the electrons in an optical-power meter. But how do we determine the energy density of light theoretically? Again by measuring it, but this time theoretically. We do a thought experiment and see what the energy content of a beam of light is by letting the electromagnetic wave be absorbed by an electron. The work done by the field on the electron, via the Lorentz force, then gives the energy density of a beam of light. I remember being deeply impressed by this finding, seeing that experiment really lies at the heart of theoretical physics.
A child growing tall is like a wavelength being red-shifted.
Now turning to the problem of measuring the height of your child experimentally. If we are to measure this length correctly, then we should have the child stand still against some background. We can then mark off the position where the head of the child coincides with the background. (Unless your child is a bird, their feet are conventionally assumed to be touching the floor and so do not need to be marked off against the background.) Subsequently, we can place a ruler or other standard unit of length alongside our markings and express the marked off distance in terms of our standard unit of length.
Alternatively, you can use a fun and colourful wall sticker with a growth chart printed on it. These wall stickers may have happy animals or other pictures on them which do not serve any scientific purpose other than inducing the cooperation of the subject. We have one with a tree and jungle animals and we have one themed after a popular fireman cartoon figure. These wall stickers can be stuck to the wall of your child’s bedroom for decoration while at the same time making it easier to record the growth of your child. For example, if you have such a chart stuck to the wall, it becomes super easy to have your child stand against it and to take a quick picture. There’s a dated record that can be shared with family straight away. What a very practical use of theoretical physics!