More information would be helpful. Are you setting up a tripod? What type of camera is it(crop factor?)? How is the lighting.
My guess is that you'll want to go with the 50mm. If you plan on shooting handheld or having a decent amount of movement, you'll definitely want to shoot with the 50mm.
Indeed, more info is required.
Are you indoor, outdoor, kids are sitting, running, are you on a tripod, handheld, and so on?
100mmL is the best lens to stabilize video so this is the way to go if you are indoor and only the kids are heard.
Otherwise, go with the 50mm especially if you are on a tripod without an external microphone.