Yes, as long as you can communicates with teachers, western people are very nice, they don't care grammar too much. Also, if your kids can understand the basic instructions, I believe it is not that worst. Children learn new languages are too easy if they are very young. If you can try, the interviewers will tell you whether he/she can fit into their schools.
韓國國際(KIS) international section interview with English (at least the present or former principal are non-Asian and do not (or suppose cannot) communicate with Cantonese.
Not only interview. If in IS must speak english everwhere. My reasons to put my son in IS: want to have a happy childhood without stress, develop an outspoken charcter. Hope this help.