Yes whoops you're correct.
Each toddler gets 3 seats. Each adult gets one seat.
Ideally each row is 3 seats.
Or, two rows of 4, its the same number of seats (8)
Thanks!
Let's assume post-covid and seats into/out of Australia are plentiful and fares are back to 'normal'. Qantas will allow you to book and confirm additional comfort seats. You
must do this through the call centre and each extra comfort seat will be charged at the base rate without any taxes fees or charges. Looking at June 2022 as an example, Qantas has return fares from Australia to London for $1817 per per adult, comfort seats would be $1450 each (no taxes fees or charges). Children 2-11 pay the same adult fare. If the toddlers are under 2 different fares will apply for the lap infant - but this wouldn't affect the price of the comfort seats.
You may run in to some issues with a toddler being in a set of three seats by themselves given the requirement to make sure their seatbelt is fastened if the sign comes on, and that an adult can assist in the event supplementary oxygen is required. It might work if the toddlers were the row in front and behind the two adult seats, but I'm not sure how QF would go to confirm this arrangement for you. If QF is flying their A380s at that stage it would be perfect with 3-4-3 seating as you'd have one adult and one toddler per row.
Other airlines such as Etihad will allow you to request additional seats, and are much cheaper, but the additional seats aren't confirmed until very close to departure.
Total cost of the QF option would be 4x1850 and 4x1450 = 13200.
However... another option would be the potential to fly via Bali. Assuming restrictions were lifted by then and all flights operating normally... you'd buy 4 x economy jetstar ($2400 for the family) combined with 4 x full business class Bali to Europe for $2880 per ticket, comes to $13920. Slightly more expensive but everyone gets a bed!