for pre-algebra get some balancing scales
first thing the child needs to know is the = (sign) means exactly the same (in all cases units/dimensions ect) for both sides of the equation, whatever is on the left side is/should be- exactly the same as the rt side of the = side in every way, no exceptions , but you have to compare apples with apples and oranges with oranges.
so there is a symmetry no matter how complicated it looks.
for young children use scales with weights in kg, grams, ounces and pounds allows them explore this idea.
no doubt G55 will come along and put me right, if so I would follow her advise