Context · YouTube
Shorts is a full endless vertical feed built into YouTube, so a family that decided against TikTok often has something extremely similar already installed on every device in the house.
It behaves nothing like YouTube
Regular YouTube is mostly search and subscriptions, where a kid goes looking for something. Shorts is a feed that decides for them and never ends, which makes it far closer to TikTok than to the app it lives inside, and the rules a family set for YouTube usually do not fit it.
Why it is easy to miss
It has its own tab and its own recommendations, and it is on the television app too. Parents who checked YouTube and were reassured by the watch history often have not looked at the Shorts tab, which can be a completely different picture.
What you can do about it
You cannot remove Shorts, but you can turn off the Shorts feed in the app settings on many devices, and you can look at that tab specifically when you check in. Even just naming it as a separate thing helps, because it lets you have a rule about a feed rather than about YouTube.