Custom Thumbnails with Advanced Custom Fields

I’m having an issue (probably “user headspace error”) with the “use_custom_thumbs” variable and Advanced Custom Fields. Reading the documentation, it looks like I needed to create an Advanced Custom Field to return either the URL or the ID of the image (I’ve tried both). I made the ID and Name of the field match the “custom_cc_thumnail” given in the example in the documentation. (I have also tried giving the field a unique ID and Name and calling that identifier in the use_custom_thumbs variable, but that didn’t work either.)

The field shows up on my page edit screens and allows me to attach the image to the page, but I don’t see the thumbnail used on the page where I include:

[child_pages use_custom_thumbs=”custom_cc_thumnail”]

It just shows the standard exerpt etc.

I have to be reading something wrong. Any ideas would be appreciated.