Was just back from the pub when I replied above…
There is a difference between 2 different types of bike:
a cross bike designed for 700c x 33mm tyres, with 650b tyres in it, which are usually going to be narrow MTB tyres or ‘road plus’, so at least 45mm wide, which as mentioned above, lots of standard cross bikes will fit and the overall diameter of the wheel ends up similar to a 700c wheel with narrower tyre. With these tyres in, it isn’t really a Cyclocross bike any more but a fun 650b monster cross thingy.
Or, a cross bike designed for 650b x 33mm tyres, for smaller riders, which is a tyre size that doesn’t seem to exist yet, but one that would make sense. The closest I’ve seen to this is the 26 inch Cyclocross bikes marketed for kids which use one of the very limited range of 26 inch narrow knobbly tyres.