This is kind of a two headed monster. Runner on 3rd: If you're on offense, why would you give up the out, knowing that your baserunner is going to be standing on second one pitch later? Why as a defender are you worried about the runner going to second knowing they will be there one pitch later? Any decent coach is going to have the take sign on to get the runner over knowing the catcher isn't throwing down with a runner on 3rd. The older the age group, the strategy might change a bit, but at 10U, unfortunately in most cases, you give up the run for the out.
Defensively, it's about getting them to do the situation right. Sure, the baserunner will probably make it down to second anyway, but having your defense execute the play when the coach calls it is much more important than stopping a girl from getting to second or giving up a run. Eventually they'll add layers to it like I saw (I think Auburn one of the SEC schools do recently) where the short stop covers the throw from home to second and the second baseman flashes across and can either cut the throw and get the runner from third or let it go so that you can get the out at second. If all you do situationally is give up the base, all the kids will learn is to give up the base. Why bother throwing down to second on a steal if your catcher can't reach? Because you want them to learn to how to defend the situation, even if they can't get the out yet.