I don’t like the twee made-up word tween which apparently means a child between toddler and teenager.
Instead, I prefer the term middle-aged child. Because it further muddies the aging process that the introduction of “adolescence,” which means not a child and not grown up and without responsibility, apparently, that leads from puberty to age forty these days. By calling them middle-aged children, we’re just begging for hundreds of Internet articles about mid-childhood crises to gull young (and adolescent-at-thirty-four) parents to solve a problem that didn’t exist before the word invented for it.
Fortunately, though, most of the time I’m just talking to myself and Young Nick, the office kitten, so this phrase won’t catch on and a psychological crisis amongst our youth will be avoided. You’re welcome.