Personally I would go with Children of Men, but I have yet to see Little Children. But here's what I believe both movies to be.
Children of Men - Post-apocalyptic move feel, with a very harsh and dramatic story to follow. The movie is very good and Clive Owen does an incredible job.
Little Children - Romantic-Drama with a tangled love story.
Both movies are probably great, and I would see with one intrigues you more, read more at:
http://www.imdb.com/title/tt0206634/
http://www.imdb.com/title/tt0404203/