You'll have to buy "tickets" for both since its international (purchased ticket for lap child is just the taxs though), one will have to be in a seat so hopefully your on an airline that will give discounts to children... some airlines are cutting that option due to "finances".
If you don't have a carrier I would suggest getting one, because as the pp pointed out the getting to the seats and out is going to be tricky. Does the older one walk well enough to get from gate to seat? And if they do I hope they can carry a little something to help out! (My son wears a mini backpack of his plane snacks).
Also many airlines have rules that FA CAN'T carry or hold children/infants... many don't listen to it but know its there and SOME DO follow it.
Not easy but Its doable! Good luck!