I think this may be an episode of "First Wave" - on the other hand, I also think it's not... that it's from another show and they were just aired around the same time here in Norway/Europe (which means they may have aired years apart in the USA).
It was an American (USA) or Canadian show. It's about (I think) a man being pursued by aliens (may be just one) - possible some warrior-alien.
In the episode, the man encounters what he first thinks are aliens in white glowing body-suits (tight cover-alls) in an abandoned building, but it turns out to be just a pair of teenagers (13-15) dressed up for Halloween/carnival. I think they're brother and sister (possible twins), but I guess they could be boyfriend/girlfriend. The man ends up befriending the kids and seeks their help.
Toward the end of the episode, the alien has tracked the man down, and creates panic at the carnival. In one scene, we see through the alien's eyes as the twins scatters before him.
This was the only episode with the twins/kids, it's just the episode I remember... I would guess the show is at least 7 years old.
I'd like to know which series this was from and particulars to the episode as well.