Is it "so ... that" expression? Should I understand it like "they're SO simple (THAT) a child could make them"?
Yes, you can understand this phrase as "they're so simple that a child could make them", and "that" is implied. "So" is used here for emphasis. Sentence structures like this can include "that", or omit it. Ex: This t-shirt was so cheap I bought two of them. Ex: The meal I had at that restaurant was so good that I went back the next day.