The Erector Set was a mainstay in my house growing up, inherited from my Dad, the original case long since discarded. It was the surefire building toy to turn to when you needed to make a hamster elevator or a robot that just sits there.
Turns out they make a set for little kids. The package says ages 4-8, but Dads with seven-year-olds get angry about its limitations, and this Dad introduced his son to the kit at the age of three. It looks like three is the magic number here, as long as a Geekdad is willing to show his son or daughter how it’s done.
Most plastic building sets for young children don’t involve this kind of a flexible system, so it looks like a good open-ended toy to me. A 100-piece set is under $20 on Amazon.