I am talking in hypotheticals here. I have never been in this position myself. I am also pointing out that there are many other people that are as skeptical as me (or even more).
if true I wonder why I've never encountered any while shooting
I understand that, but if I was at an event and someone just didn't seem right to be there I would ask around first to see if anyone recognized them. If nobody did then I would have no problem asking them their reasons for being there. It all depends on the venue as well. A football game with hundreds of people there is not the same thing as a gymnastics practice with twenty parents there.
a practice inside, should be more of a controlled environment I would think the people running the place should be on top of things
Like I say above it does depend on the venue and situation. It would never be considered harassment to question someone's intentions. If it went into threats of violence that is a whole different story and not what we are talking about here.
if you ask one question no, if you persist and keep them from doing what they are there for , it could be harassment.
By that I mean not welcome to shoot my children. Again it all depends on who, where, event type, if permission was given by organizers, etc. The thing I was commenting on originally was that the photographer Geoff_M knows is automatically confrontational when asked. In my experience, someone that is automatically confrontational is usually doing something that they know that they should not be doing. A professional would never act that way and would easily be able to explain why they are there. You are probably a potential customer anyway, so they would not want to make you mad. If it is just someone with a new DSLR practicing, then they should have no problem saying that they will not shoot your child and be done with it.
I didn't get that impression of the guy Geoff mentioned, if his comment always diffuses the situation, obviously he doesn't come across as confrontational to the parents involved..
Me personally, I am not worried about that. It would more likely be me being questioned by someone else while I am photographing. Also, while I am a really friendly person, I have a somewhat intimidating appearance being a bodybuilder. People typically do not get edgy with me.
and that attitude is confrontational in and of itself, if I was approached by a body builder type, I would not be intimidated, especially if he apeared to have the attitude that some bodybuilders have, I'd be quite comfortable knowing the law was on my side, and knowing that if he touched me I'd own him 

(Comes in really handy with teenage boys interested in my daughter

)