Request a child's meal,they are always good for children.
I have done the flight from Australia to Spain for the last 5 years and have never had any problems.It is a long flight but I found my DD to sleep most of the way.
I always take dried biscuits on the flight too,like milk arrowroot biscuits or anything similar.
If you have a stop half way,get lunch at the airport if you can,as that fills them up too.
Have you been to the doc for some stuff to help them sleep?
Good luck hope all goes well..