child, servant, foolishness
僮 centers on a young person in a subordinate role: a child or servant. The foolishness sense extends naturally from the idea of youthful inexperience.
僮 combines 亻 (person) with 童, which likely contributes both sound and the idea of a young person or servant.
A person 亻 beside 童 (child) suggests a young servant or child. The foolishness comes from the inexperience of youth.
For トウ, imagine a young servant carrying a tall stack of towels: towel -> トウ, and the child struggles under the load.