IGCSEs are taken around age 16 and IB at 18, so they don't have much relevance for your 6 year old, who is at the infants/primary stage. Some IB schools follow the Primary Years Program (PYP) which is a method of teaching rather than a prescribed list of things to be covered. You don't say when you need a place but that will determine whether you have much of a choice. Best bet is to contact all the
International Schools and see what is possible then do your research on their websites re curriculum and if possible, visit and get a feel for the schools. Some are huge, others smaller or have multiple campuses. Will your child have to move campuses at a certain age? Will that affect where you choose to live? Some have only or mainly english speakers, others higher numbers of ESL children, which may give you a bigger international and cultural mix. You have to ask yourself what environment will best suit your child. The schools list info on fees on their websites or contact them direct.