Use luggage cubes and mix the outfits into different luggage. Even if one suitcase is lost everyone has an outfit.
Packing cubes are great, with bands they're so much better because your clothes stay together. It's also a space saver!
Someone is bound to get sick, or have an accident. A travel first aid kit with the basics is essential.
Headed to the beach or pool? Pack a wet bag to keep all of your wet swimsuits away from everything else.