A short bike stem is common, and desirable, on MTBs (increased maneuverability) and a long bike stem is common on road bikes (lower body position), but is an ultra-short bike stem downright dangerous on a road bike? If not downright unsafe, is it safe with a small group riding in single file, but becomes unsafe for someone riding with a crowd?
Context of the question: A road bike of a given size will typically last a growing youth one season (up to 6 months) before it becomes necessary to replace it with a larger bike. One way to somewhat reduce the rhythm of replacement is to have a collection of stems (40mm, 60mm, 80mm, 100mm, 120mm), and to use one after the other. (This can be combined by buying the bike slightly ahead of time, and only allowing the child to ride when they clear the top tube.)
In case it matters, this would be the youth's only road bike. I appreciate that there are some motor skills that could get confused if one is switching between two bike frames. The magnitude of the "input" for a long stem, if done unconsciously (reflexively) could lead to excessive steering on a bike with a short stem.