We have a trunki for each child, but we only got them recently. We're a fairly high-travelling family, and I've always just put their stuff in our suitcase. If we plan to be separated (for instance, different rooms, or we plan to take a one-night off while they stay with grandparents) we take another small suitcase. Here are a few things we found useful:
1. if you're taking bottles/sippy cups, stuff some clothes inside, it keeps the plastic from breaking under pressure and saves space.
2. pack diapers for a full day, and then take a couple extra. We thankfully never had to use all of them, but I always worry the plane will get delayed and we are stuck on it for hours and hours.
3. iPads or other electronic gadgets are magic for long flights if you can afford them.
4. When the kids were too young to pay for a seat (under 2 years), I always carried them in the baby-carrier. It's a very strange environment and being close to mum helps. Also, frees your hands up to deal with the other one
5. nurse/give bottle/water during take offs and landings, it helps with the air pressure ear-ache.
6. Don't over-pack. I always find this really hard, but it's much more practical to just run a laundry (or wash a few clothes in the sink) when you're there, especially if you're going for over a week. Of course, you should fill the suitcase(s) you ARE taking to the limit (the fuller it is the less likely things are to be jostled).
Good luck, and don't hesitate to ask specific questions if you still have them
