When I was younger I read a series (maybe a trilogy) about some children who lived inside a sealed off city in futuristic earth. One character as I remember was real daring so he convinced one or two others to escape the city with him. Once outiside, they were searching for something and encountered a semi-post apocalyptic environment. Every so often they'd run into some vagrant humans. They also were chased often by these "tripod" mechanical creatures (human-controlled?). I forgot what happened in the end, but it was an exciting series and would be sooo happy to know the author or any ideas. Thanks ahead of time for the help. 