In this paper, we investigate the duration, lexical and linguistic properties of children's spontaneous speech for children ages 8 to 14 interacting with animated characters in a computer game. Age and gender trends are studied for parameters such as phone and sentence duration, speaking rate, °uency (mispronounciations and hesitations), vocabulary size and linguistic variability measured via bigram language model perplexity. The analysis shows significant differences between read- and spontaneous children speech in terms of absolute values of acoustic and linguistic parameters, as well as, linguistic variability. In addition, spontaneous data present clear gender-specific trends, e.g., increased "language exploration" by girls in the 12-14 age group. The applicability of these results for acoustic and linguistic modeling and spoken dialogue systems interface design is discussed.