a 7-month old can sleep thru the flight without much interruptions, they DO provide bassinets, book early and make sure u request for a bassinet, also dxb airport provides free strollers as soon as u arrive at the gate, there is a small playarea in t3 where u can goto and maybe ur kid can unwind after a 15 hr flight.

when ur kid is 18 months and above thats when the mayhem starts -from my own daughter's experience

the additional 5k is upto ur means, biz class is definitely the way to roll on a 15 hr non stop. but me, my wife & daughter, we have travelled the iah leg in economy and it wasn't unbearable. remember bassinet seats in Y are at the bulkhead, the armrests are not movable so u wont be able to stretch out sideways.