For a child beginning algebra, it can be useful to introduce the subject as a kind of “code”, where you can substitute numbers for the letters.
Then you can move on to constructing your own “codes” for ideas the child should already be familiar with – miles per gallon, speed/distance/time. This prepares the child for the idea that algebra can be used to create general rules that suit all situations. These can be expressed in words (speed=distance/time) or letters (s=d/t).
I’m not sure what 12yo translates to in grades or years in school. However,has lots of videos and tutorials suitable for people of this age, and the student can track their progress.