We had our first holiday as parents in October last year with our almost 3 year old and 6 month old.
It was a 4hr flight and the baby was no bother, either fed, slept or cuddled me or DH for the whole flight.
For DD1, we gave her her own backpack to carry on which contained:
Snacks
A change of clothes (mid potty training)
iPad and head phones (downloaded her favourite cartoons and some Disney films)
Her water bottle (filled once airside)
A small pack of colour pencils & a colouring book
Her baby rabbit cuddly toy who goes everywhere with her
She spent the whole flight just flirting between watching her iPad, chatting to us and eating snacks.
On the way home it was a night flight so she watched her iPad for a little bit before cuddling in for a nap.
We also took calpol sachets in both mine and DH's hand luggage and had a nappy & sleepsuit, wipes, knickers, leggings and t shirt in each of our bags so whoever had the baby or toddler could just take them to the toilet without having to rummage around another bag. We found it worked really well doing it that way. Just popped them in a zip lock bag and wrote their names on the front.
We also had huge muslins which doubled up as blankets, pillows, swaddle, etc