Its not easy to travel with infant; but following will certainly be helpful:

1. Since both of your kids are 5/6 months, it is not advisable to have extra seat for infant. Infant will be on your lap.
   
2. Carry stroller (light weight / folding type) along with you. In any int'l flight, it is allowed (like laptop, over coat, umbrella, no extra cost/weight). It will be with u inside the cabin space.
   
3. VERY IMPORTANT: choose the infant row seat (look at the seat plane of aircraft), the air-craft provides bassinet. So, when baby sleeps, you will be using the bassinet; if not s/he will be on your lap. Note that you have to book in advance for this seat to avoid future circumstances.
   
4. Since you are traveling with your baby, do not go for extra hand bags. A small bag where baby food/wet wipes/few nos. of diapers, etc.) is must plus extra baby clothes.  Try to send as much in checked-in luggage.
5. Since you are carrying stroller, u can use it during transit. It will be too convenient.

>>>How about liquid/powder baby formula and cereals?

Carry sealed water bottle and later u can open once u pass immigration). Anything that is sealed is ok. Baby formula/cereals: if they are sealed, no worries. U may buy the smaller one just to use during the flight/travel.