For the Flying + Airport part (if you are)
I travelled solo with a 4 year old, 2 connecting flights, 2h + 12h earlier this year, and my advice to make it as simple as possible is:
Child takes a backpack just full of stuff to do while travelling (you manage what they bring), that includes things suitable for: Looking at while standing in queues, Looking at during take off + landing, playing with on a table during flight. Kids are responsible for carrying their own toys/stuff
If possible get/have suitcases that they can sit down on during any wait times.
Bring food that is fiddly/takes time to eat (lots of small pieces etc.) great for boring moments.
Buy something new in the airport for the flight - the excitement will ride over a lot of the boring parts of airports/journeys.
For the 4 year old - consider a foldable tray. I had one that could fit in my backpack, then could fold out so that it had sides. Put it on his lap or the flight table = no toys always falling off the table under the seats.
Make sure they have their own backpacks, and maybe even small suitcases, give them responsibility and let them take pride in getting through each part of the journey.
If it is a long flight and you want them to watch things - prepare comfortable over-ear headphones, and any adapters needed to connect to devices/airport screens.