While many of the individual tips would work here as well, here are a few tips more specific to working with groups...
Unless you just absolutely love the idea of having everyone in the same outfit, I don’t recommend it for families. Avoid anything too formal (ties, dress shoes etc) so that everyone’s comfortable and be sure that no individual outfit stands out more than the rest.
As a general rule for adults and older children, keep all the clothing solid (no patterns) in one color family (earth tones work well as do schemes such as blues and orange or pinks and reds) but limit the number of different colors. We want the focus to be on the people and not the clothing. An exception to this rule would be to have only the mother wearing a print but nothing too very bold.
The best advice for dressing children is to let them be themselves and look like kids. Anything too formal will only make the child uncomfortable. Hats & scarves, boots, bright colors, hoodies, layers, sundresses, pigtails, rolled jeans, bare feet, and even tutus work well. Jeans and overalls do not work well on babies.
Avoid dressing anyone in white if possible (off white and light pastels are better).