It's probably too late for this weekend, but I would have thought you want a selection of material where they are playing to the camera and where they are not. I don't know how much you get to see them, but if you get the camera out every time they'll soon become accustomed to it and begin to ignore it, giving you much more opportunity to capture the children acting as they normally do. These shots are precious.
I have a 7 year old and she'd grown up with me brandishing a camera (I got into video simply to capture her growing up - something I regret not having done with my other two - now adult - children). The camera is virtually invisible to her unless we are filming something which she is specifically performing.
And that's where Ned ideas comes in. If they insist on perfroming for the camera, this is best if you are recording a specific performance (eg a dance they've made up or learnt) or a story (and it cam be great fun working on the story together). These are likely to produce much better footage than the children simply showing off.
If you can tie it into a school project, or even just a subject they are interested in, it can be educational too. Over the Easter holiday she had to produce a poster, leaflet or other presentation about one of the subjects she'd been learning. She'd really loved the story of the Fire of London and I suggested if she wrote a script, we could film it. The result was (a) she got here homework done, (b) she consolidated her knowledge of the Fire, (c) I got to experiment with mucking about with still drawings and photo-editing software (d) I had an excuse to learn how not to use a green screen (e) my wife didn't mind me filming because I was entertaining Elise and (f) we have some film of Elise that we'll hopefully still be able to enjoy in many years' time.
Maybe you could think of something (or,better still, get them to think of something) and work on it together with your grandchildren via email in advance of the next time you see them.
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Ok so here is the problem...ive taken some footage of the grandchildren tonight... blimey 
don't they move fast 
but I'm off to see my daughter in a couple of weeks where my other grandson wants me to film him for his dads football website... ive told them to put together some ideas and props