Hi I'm Mongolian citizen holders. But my two children is French citizen holder

soon we will go to Mongolia but we will stop in Beijing in night I can't catch even last fly or train to go to Mongolia so I need sleep one night in Beijing and maybe we will take train so do my children's need visa or that 72h is ok for them? Also we will take emirate from France to Beijing emirate accept 30kl,, I read in Internet beijing to Mongolia accept 20kl how I do this situation??? Pls