The 24- 70 should work fine. I read this as you making a video, some sound like they think its still photos.
You should be clear, so you get applicable advice.
Handheld video is very difficult, it can be shaky and boring. Mount the camera on a tripod, or even sit it on a stack of boxes or other support if you can. A monopod is another option. You also have to manually focus.
You cannot zoom the lens and expect it to stay in focus, lenses are not parfocal until you get into the $40,000 range. Zooming a perfectly focus lens will result in out of focus children. If you are close to your subjects, using a wide aperture means a shallow depth of field, and some of the children will potentially be out of focus. Figure out your focal length and distance in advance, that will help you decide the aperture and ISO you will need.
I would plan out your shots, measure the distances from the subjects and the lens focal length required. Then try to determine the amount of light, and see if you can set the lens at f/4 and ISO 3200 or less. If that works, then you can use any of the lenses. At f/4 and 6 ft from your subject with your lens set to 35mm, you will have just over 2 ft depth of field. 1.28 beyond the focus point and 0.9 ft in front of it. At f/2.8, the depth of field will be much less.http://www.dofmaster.com/dofjs.html
Work all the things out in advance, and put down some masking tape or other marker on the floor or ground there you want them to stand. Same with the camera. If you just expect to walk up and start shooting, and you don't have a 2nd chance, you need to control as many factors as you possibly can, and have a idea for plan b if that doesn't work out.
Post more information are you doing video? I took the word filming to mean video, but its not certain.