We generally never pack clothing for more than 5 days and plan on doing laundry along the way. We stay at airbnb apartments that usually have a washer on site and a kitchen. This has transformed our traveling experiences with children.
We prefer to use backpacks to keep our hands free for wrangling and carrying our children. Roller bags are good but you lose a hand and dragging it through Europe can be extra difficult. They call it, "backpacking through Europe" instead of "roller-bagging through Europe" for a reason.
Most of my time packing is spent during the pre-packing phase - taking the time to consider the types of events (fancy dinners - haha, these are family packing tips. Nobody's enjoying a fancy dinner for a while), modes of transportation (train, bus, car), city vs country landscape, number of days and the weather for the trip.
We pack our clothing into our favorite Kelty backpack (large) and our immediate necessities into our medium backpack. We keep bribes/toys in a handy front pocket for the inevitable meltdown after a 17 hour travel day. I prefer the rolling method for packing clothing:
Tips for kids and babies
We also only pack enough diapers and snacks for the travel day and buy diapers and other large liquids (e.g., sunscreen and toothpaste) on site.
Kids and babies look cute in everything and can get away with wearing the same thing day after day. Most days, my 3 year old demands to wear the same shirt day after day so there isn't a need to pack a gazillion different outfits.
I wear our 10 mth old in a baby carrier, the Ergo, and she and our 3 year old swap out time in our travel stroller. It folds up easily and is super light.
If your toddler will wear a backpack, put their light toys and books in that. Time to earn your keep, kid!
Tips for women
Pack colorful scarves to dress up plain tops and/or to cover your shoulders if visiting religious sites or fancy dinners
Pack tops that can mix and match with all of your bottoms. Steer away from "outfits" that only match each other. Being able to recombine your clothing means you don't need as many separate items.
Never pack one-off clothing or even worse, clothes you may never wear on your trip. Your goal should be to wear every article of clothing at least once. Space is wasted packing for events that are unlikely to happen. Do you need those bulky rain boots for one day of rain? Probably not.
Go light on toiletries and makeup. My hair never looks the same as it does at home - the water and humidity are different so I don't worry about hair products. Hotels always provide those little soaps - just use those or pack a small bottle of conditioner. My hair can withstand any shampoo but the right conditioner helps to keep it tamed.
Tips for men
Your clothes are probably the largest in the group so be strategic in your selections
Mix and match your tops and bottoms
Pack only one pair of shoes
Wear your largest, bulkiest clothing and shoes on the plane
Baby cribs can be checked for free along with your travel stroller (on Norwegian anyway). You can stuff some small things into the baby crib bag if you need.
Print off all tickets, itineraries and receipts before your trip and place them in a folder. I print off our overall itinerary (day by day) and tape it to the front of the folder for easy referencing train coach numbers, seating arrangements, etc. I get very nerdy and label everything with sticky notes that are in chronological order for when we will need them.
Remember, the less you pack, the less you need to keep track of and repack at the end of your trip.
I know "pack less" is kind of a flippant tip but it really is an exercise in restraint. I used to pack "something for every occasion" or an extra "just in case" item but found that it was rarely ever used and just took up space. Most airlines charge for checked luggage so traveling lighter saves you money that you'd rather spend on dessert or wine.
At the end of the day you aren't going to care about what clothes you wore on your trip. However, lugging less stuff means you are physically less tired, resulting in less stress during your travel. It will enable you to enjoy your trip while you are actually on it.
Now, if only you could pack an extra pair of hands that would be great! Enjoy your trip!