Yeah, happened to me a few years ago some times. Stupid pricks, stalking kids...
The first time, I was about 11 years old and this asked me if I had money in my wallet and if he could see it. Of course that's about the most stupid way of trying to mug someone, but I was really young so I just showed it to him and he grabbed 10 euros and headed off. Luckily, when he tried to do that again some months after, I just told him to f**k off.
The second time, I was coming home from school with a friend when these two guys started following us. Near my building, they stopped and told us to hand over our money. My friend got headbutted in the chest and I got away without a scratch, and they got like 2 euros, and we just ran home. Scary stuff, but since then I've been able to avoid the groups that hang out in that area and the next two times someone started to follow me, I caught their reflection on the windows of a gas station, walked in and waited for a while, after informing people I was being followed, and fortunately nothing happened after that.
Actually, I guess I've been pretty lucky seeing as I've never been hurt and they only took smalls amounts of money. But it was all still pretty scary, especially as a kid.