You can compose a collage around:
Content, like the sailboat collage above.
Color, like I have in my apartment..
Or even a looser theme, where this person used neutrals and large mattings.
I think these make even more of a statement than a regular collage and are also easier on the eye. It's also important to note that if you want a more cohesive look, be aware of the medium in which the art was created. If you are creating a family collage, are you using a collection of photographs and drawings? Maybe it could be more interesting if you only did all pencil drawings by your child, rather than to combine it with photography. Like most things, simpler is better, even with collages. :)