What to wear?
Classic: Timeless style, chinos, khakis, beige and blues, nature-hues, pastels, shirts, and simple dresses. The flowy-er the better IMO.
Florals and prints: Just florals, or an eclectic mix of prints and patterns. Pairing them with stripes can also work sometimes.
Colors: Keep them complementary as opposed to completely matching (e.g. all white shirts and blue jeans). Avoid stark contrasts such as green and orange/red together, and yellows and purples juxtaposed. Complementary colors are more like warm tones (yellows, oranges, pinks, warm red and even warm greens) together and cool tones (blues, purples, greens) together. But an explosion of bright colors could also work, although I’d shoot it on a plain background or setting.
Consistency: Avoid extreme differences (e.g. one person is wearing a casual knitted chunky sweater and the other is wearing a nice silky dress). It can be quite jarring. Black and white is another combination that’s too stark a contrast unless it’s done intentionally. Avoid wearing the same colors as your partner or dressing everyone in the same color. It can cause you to disappear in one another, leaving little to no room distinguishing where one person's waist starts and the other ends.
Dark, light and bright: Darks for adults, and lights or brights for small children. Do it the other way and the adults will dominate the scene and draw the viewer’s attention, while the smaller people will disappear.