All I can remember from this was that it really freaked me out when I was seven, we watched it in class once- it may have been an educational thing, so possibly made by the BBC, but then again it might have been American... judging from the multicoloured clothes, I think it was made in the 80's/90's- that was what I thought at the time anyway.
I think some kids find an alien (kinda grey and fat-ish in appearance- obviously a man in a suit), and the alien has some kind-of device in his hand that's counting down, like a bomb.
There may have been another (bigger?) alien searching for the one the kids found, because I can remember one of the boys commenting "oh, the big alien's searching for him!", but this may have been a misinterpretation.
Towards the end of the first episode, I can remember the camera cuts to an/the alien that's on a beach littered with rubbish, and a bridge behind him. The alien's looking at the watch/timer in his hand and it's beeping dramatically before it cuts to the credits.
I think we watched more, because when the next episode started up I remember feeling annoyed at the anti-climax, but I can't actually remember what happened.
There was something about the kids going into a forest too, and in some kinda cabin (although it might have been a spaceship), there were loads of buttons and lights and something spinning on the wall behind them (which I stared at for a while and entirely ignored the conversation the main characters were having).
I can remember there were maybe five kids/teenagers, one blonde girl wearing a light-blue t-shirt, maybe some boy called Billy with a red hat (but I'm not sure)...