Having two strollers is not very practical - you tie up both parents with strollers so you normal carryon in that case would have to be a backpack as pushing a stroller and dragging a roller behind is very difficult.

We traveled quite a bit with out double mclaren triumph both in us and overseas and found it to be very practical for young twins but this may not the case if you have a 4 yr old. Try it in store to see how kids fit in it - 7 mo old may be too young for it and 4 yo too old.

I would consider a backpack carrier (Kelty or similar) for 7 mo old for dad to carry and let the other one walk and just manage your pace. You can always buy a cheap umbrella stroller for $20-50 pretty much anywhere you go if it becomes a necessity.

Another option is "sit and stand" or smth similar designed for kids of different age - some of those accommodate a basket for infant and still allow an older child to sit (or stand) when you push it.

AKL, MEL, FRA are well covered by public transport, BKK has cheap taxis and you can easily rent a car in HNL and YVR if you need to get around. I think you'll manage just fine.