What I do is I first get them all watching TV so they are already all in the same room. To do that, just pick them up one by one and drop them in front of the television and they'll find a spot to sit and watch it.

Once they are all sitting, give the movie to one of them. That will cause that one to get up and walk towards the t.v. and then they will all watch the movie together. It's supposed to raise happiness, right? I haven't noticed any real difference, but that might just be something weird in my game. If you are trying to make a kid happy, try praising it for playing, give fruit and candy to the child, you can also drop one of the parents on the child and they will either tickle the child or they will read a story to the child. Slowly but surely, the childs mood will improve.