I'm just about to do another solo return flight from Oz to the UK with DD & I think you should go for it. With advance planning of the flight I don't think it will be so bad that it would cancel out the holiday.
If it were me I'd probably try to book the seats in the back row (in advance) (or actually, the same applies to the bulkhead seats too, then you could get a skycot too, not sure if your ds2 is too old for this though), that way there is a little bit of space behind for little legs to stretch, plus you could do nappy changes in the toilet and be right next to the older two. And you'll be very close to the cabin crew during the flight.
Which airline are you flying with? Would your DD be ok with wall-to-wall childrens tv during the flight to keep her occupied? When my DD was 3 she was ecstatic to have unlimited tv on flights.
Do you have a choice of airline? Some are much better than others with children.
I normally pack lots of snacks and food and some new toys wrapped up in wrapping paper to bring out when boredom strikes. I always pack myself a mini bottle of wine too to break out in emergencies, especially as you can't drink tea or coffee with a baby on your lap .
Don't forget to pre-order the childrens meal too, it means you'll get served first and is a novelty.
I found flights where I took DDs carseat a load easier for me, but it did mean I had to pay for another seat - on the very long-haul ones I did, on shorter ones I didn't.
Have a great time! Think of their little faces if you go to Disney