Actors say you should never work with children or animals. It was once thought that it was because they would steal the thunder from the actor but it is really because they are so unpredictable that the actor was more likely to be made to look a fool.
That is why it is so much fun filming children and animals for your funny movies. Look at those funny home movie shows. The majority of the movies feature children, animals or both.
You can choose a scripted movie where you sit down with the children and decide what they are to do and say. You could just let them play and standby with the camera. You could do a little of both. Get a simple script and have the children act it out. As they are acting, you may find some priceless footage amongst the average.
You will have to be patient as the best stuff usually happens when you have just run out of batteries or have to put the camera down to save someone from damage. I am amazed at the people who keep on filming even when they see their child about to come to grief. It looks funny on camera but I hate to think of the bruises some of the children sustain just for the sake of a laugh.
It is fun to film your pets when they are playing and do not realise you are watching. You can take hours of footage and only end up with a few minutes of funny footage but when you get that footage it is golden. The beauty of digital cameras and computers is that it costs you nothing but your time.