Think you underestimate the certification requirements for welders. A simple welder certificate holder does need basic math skills. Required classes may include advanced mathematics, metallurgy, blueprint reading, welding symbols, pipe layout and a welding practicum.

http://study.com/welding_education.html

If the OP's child can not multiply, or follow ordered operations, it may be a huge stretch to to follow welding step-by-step assembly instructions.

The higher level welding certifications have chemistry and physics knowledge requirements.

http://www.aws.org/library/doclib/CWEng-bok.pdf

Source: Dad welds and has attempted to teach me. Only marginally interested :)