Make sure you have their favourite blanket and cuddly packed in a bag ...so can produce it as soon as you are seated.
If your airline is any good ...they'll let you on first ...so you've time to settle and get organised before the rush...so make sure you go through the gate as soon as you are allowed.
And let them work off as much energy as possible whilst in the airport ...don't carry/push them everywhere.
Have recently done several long haul flights with babies/toddlers on board ...they've all been very good ...so ask cabin staff for advice too!
Things you find stressful that other people don't notice.
New house and a sloping garden