This is the primary reason why I don't advocate change ups for young kids.
More often than not, change ups at this age are not a whole lot different than their fastballs. That's assuming a kid with small hands can grip the ball correctly with the middle, ring, and pinky fingers consistently.
A few reasons; mainly the younger kid's fastball isn't all that fast, which makes a change up just an easier pitch to hit for the opposition.
At a young age a fastball is more than enough PROVIDED it can be thrown as a strike consistently. Psst... strikes are good.
Change Ups don't usually become a "weapon" until the fastball becomes fast enough to allow the change up to be disguised as a fastball, and that usually doesn't happen until the mid teens or later.
That being said, I absolutely advocate practicing the Change Up grip (OK Change) on a regular basis so it isn't an awkward learning curve as the youngster becomes stronger. In fact, while playing catch, I would say mix in every 3rd or 4th throw with the change up grip.
soon enough, it will become ingrained without even noticing.