A tube kid could be a boy or girl, usually in their teenage years but sometimes well into early 20's and even beginning around the age of 11, who goes to public park, usually at night, to participate in sexual acts with one or more persons, because that person would be unable to do at any other location, such as one's house.
The term "Tube Kids" comes from teenagers who do sexual things in the big, plastic tubes that little kids crawl through and bigger kids climb on top of.
Joe: Yea guys, so I was walking Donna home last night, and as we passed the park, I heard some weird sounds and heavy breathing. I have no idea what it could've been.
Dan: Dude! Those were 'prolly tube kids!