When your toddler has family in several continents, it seems inevitable that they will be introduced to the experience of flying at a much younger age than their parents were. In our case, I didn’t leave the ground in an airplane until I was 19, and then I jumped out of it before it landed! Our little boy has already done three trips, including two long haul international trips, and he's only just 18 months old.
My first tip would be to plan a test flight, somewhere close (say 1-2 hours away), just to see how well your toddler reacts to being on a plane. We were lucky in that our little one slept both ways on the short flight we did (Oakland to Las Vegas), but in case it is a nightmare experience, you don’t want it to be too far to get home.
Take food & drink for your toddler. At the very least, you should probably have milk for the take off & landing parts of the trip so they are drinking as the pressure in the cabin is changing. Be careful not to start too soon though as you don't want them to finish the bottle before you're off the runway.
For longer flights, bring some food. We have a couple of small thermos containers that we use to keep food warm for him. Virgin gave us food for the little one on our trip over to the UK, but United did not include any meal for a toddler sitting in our lap. Either way though, having something you know your little one will find familiar and enjoy is probably a safer bet than relying on getting something on the plane.
Obviously, you’ll want to take diapers, wipes and other supplies with you for the plane. Whether you pack more for your trip probably depends on your destination. For the UK trip, we packed only enough for a couple of days, planning a trip to a supermarket when we arrived to buy more locally. Fot Tokyo though we opted to take enough for the full trip so we didn’t need to find any in a country we were less familiar with.
In the ideal case, your toddler will sleep for most of the flight. Back in the real world though, you can expect to spend much of the flight entertaining a toddler in a cramped space, or walking up and down aisles. We took a mix of new and familiar books, a handful of new toys and our trusty iPod inside its Fisher Price toddler safety cage filled with his favourite videos. Avoid any favourite toys in case they get lost en route. We took three new hot wheels cars on the last trip since cars are the current favourite; they were dropped on the floor of the plane many times and were not always simple to track down as they rolled towards the rear of the aircraft.
We also had most of his videos and games loaded on the iPad as a backup, and we had a fully charged external battery capable of charging iPods, iPads and phones with us too (that is handy always in case we forget to charge his iPod!).
We also took a selection of his books with us, including one new one from the Good Night series that he has been hooked on for a while now (ever since we got him Good Night California from a park gift store; since then we have added quite a few to his collection, and almost all of them have been an instant hit with him).
We kept it to smaller board books to make packing the hand luggage easier, but you want something that will keep your little one engaged (and quiet) for a while, so if they’re into a larger book at the time you travel, you’ll probably have to work with that.
Airport security was surprisingly straight forward. Check the rules before you fly for all countries where you will be passing through security at the airport. Our milk has been acceptable everywhere, although in the UK they made us open a random number of the formula packs we were carrying, making them much harder to carry (tip: take extra empty bottles in case you need to open pre-made formula bottles).
Our trips through security were slower for sure, especially when travelling with our oversize UppaBaby Vista stroller (which does not fit through the x-ray machines at the checkpoint), so make sure you arrive at the airport in good time. Everything went pretty smoothly though, and taking the formula (for our first trips), regular milk (for the more recent one) and food through was not a problem.
Strollers & Car Seats
One of our dilemmas for our most recent trip (to Tokyo) was whether or not to take the large stroller with us, or settle for a much more airport friendly umbrella stroller. In the end, we decided we would need a stroller he could sleep in for days when we were out at nap time, so we took the Vista. We already had the travel bag for it though, which I would definitely recommend (make sure you practice putting the stroller in it once at home though so you know how it works). We did not take a car seat as we did not rent a car in Japan, and traveled by train everywhere we went not by car at all.
For our earlier trip to the UK, we were lucky enough to be able to borrow my brother's stroller that out little one's cousin had outgrown, and one of his car seats too. The down side to that is that we did not have a stroller at the airport for him to sleep in, or even just to aide in getting him through the airport, but it was one less thing to deal with.
If you're planning to rent a car at your destination, you might want to consider checking the car seat too so you have a familiar car seat (to both your child and you).