Seriously, if your (meaning the OP) letting them run around freely to the point where you are thinking of getting them an iPhone to track their locations at ages 5 and 8 you might wanna take a look at what YOUR doing wrong.
My daughter is 7 and pretty social and no way do I need to 'track' her whereabouts. If I have any questions, I ask the parents "How they doin?" and that's that.
Now the kids across the street (sounds like your kids) they'll end up on milk boxes (yah I know they don't do that anymore) but at 5 and 8 and bf's? I know all my daughters classmates and other friends and the last thing on their mind are boys. Mostly it's iCarley or Hannah Montana. LOL
And yes I did catch the 'joking' part but to even suggest thinking that at ages 5 and 8. When they do reach the age of 'spying' on them, there's bound to be a much better way to track them.