Look at Dusseldorf: more choices and a nicer city.
Some friends of ours that are from the Cologne area arent really high on St Georges FWIW. Their rec if you want the int school route is
Homepage of ISD: International School of Düsseldorf e.V. or just put them in the local schools if they are young enough..sink or swim.
Also last time I looked, there were a few private schools in the area that conformed to the German curriculum and thus received vouchers from the Gov per student which resulted in the tuition being about half of other private schools.