1. What type of bicycle? Road bike, mountain bike, beach cruiser, hybrid bike – what?
2. Terrain? Off road, on a paved surface, a little or both – what?
3. Wind speed & direction? Even a mild 5 mph headwind can slow a cyclist down. Conversely, a 5 mph tailwind can add a couple of mph.
4. How active or athletic is this “10 yr. old kid”? If it’s your everyday, run-of-the-mill, couch potato, video game playing kid…not very fast.
5. On the other hand, I’ve seen kids get into races called criterium or “crit races” sanctioned by USA Cycling that could kick butt.
In short, there are too many variables. The average speed of most cyclists is only about 10 mph. You may as well ask, “How fast can a 10 year old kid throw a baseball?”